ValExtLinks uses the Unix tool 'curl' to query each link. These are the return values which can come from 'curl', taken from its man page. These codes appear in parentheses after the three-digit [[../HTTP codes|HTTP response code]] for each link (except EI and IW links). 
EXIT CODES
       1      Unsupported protocol. This build of curl has no support for this
              protocol.
 
       2      Failed to initialize.
 
       3      URL malformed. The syntax was not correct.
 
       4      A  feature  or  option  that  was  needed to perform the desired
              request was not enabled or was  explicitly  disabled  at	build-
              time.  To  make  curl able to do this, you probably need another
              build of libcurl!
 
       5      Couldn't resolve proxy.  The  given  proxy  host	could  not  be
              resolved.
 
       6      Couldn't resolve host. The given remote host was not resolved.
 
       7      Failed to connect to host.
 
       8      Weird server reply. The server sent data curl couldn't parse.
 
       9      FTP  access  denied. The server denied login or denied access to
              the particular resource or directory you wanted to  reach.  Most
              often  you  tried to change to a directory that doesn't exist on
              the server.
 
       10     FTP accept failed. While waiting for the server to connect  back
              when  an active FTP session is used, an error code was sent over
              the control connection or similar.
 
       11     FTP weird PASS reply. Curl couldn't parse the reply sent to  the
              PASS request.
 
       12     During  an  active  FTP  session while waiting for the server to
              connect back to curl, the timeout expired.
 
       13     FTP weird PASV reply, Curl couldn't parse the reply sent to  the
              PASV request.
 
       14     FTP  weird  227  format.	Curl  couldn't	parse the 227-line the
              server sent.
 
       15     FTP can't get host. Couldn't resolve the host IP we got  in  the
              227-line.
 
       16     HTTP/2 error. A problem was detected in the HTTP2 framing layer.
              This is somewhat generic and can be one out of several problems,
              see the error message for details.
 
       17     FTP  couldn't  set  binary.  Couldn't  change transfer method to
              binary.
 
       18     Partial file. Only a part of the file was transferred.
 
       19     FTP couldn't download/access the given file, the RETR (or  simi-
              lar) command failed.
 
       21     FTP quote error. A quote command returned error from the server.
 
       22     HTTP page not retrieved. The requested  url  was	not  found  or
              returned	another  error	with  the HTTP error code being 400 or
              above. This return code only appears if -f, --fail is used.
 
       23     Write error. Curl couldn't write data to a local	filesystem  or
              similar.
 
       25     FTP  couldn't  STOR  file. The server denied the STOR operation,
              used for FTP uploading.
 
       26     Read error. Various reading problems.
 
       27     Out of memory. A memory allocation request failed.
 
       28     Operation timeout. The specified	time-out  period  was  reached
              according to the conditions.
 
       30     FTP  PORT  failed.  The PORT command failed. Not all FTP servers
              support the PORT	command,  try  doing  a  transfer  using  PASV
              instead!
 
       31     FTP  couldn't use REST. The REST command failed. This command is
              used for resumed FTP transfers.
 
       33     HTTP range error. The range "command" didn't work.
 
       34     HTTP post error. Internal post-request generation error.
 
       35     SSL connect error. The SSL handshaking failed.
 
       36     Bad download resume. Couldn't continue an earlier aborted  down-
              load.
 
       37     FILE couldn't read file. Failed to open the file. Permissions?
 
       38     LDAP cannot bind. LDAP bind operation failed.
 
       39     LDAP search failed.
 
       41     Function not found. A required LDAP function was not found.
 
       42     Aborted by callback. An application told curl to abort the oper-
              ation.
 
       43     Internal error. A function was called with a bad parameter.
 
       45     Interface error. A specified outgoing  interface	could  not  be
              used.
 
       47     Too many redirects. When following redirects, curl hit the maxi-
              mum amount.
 
       48     Unknown option specified to libcurl.  This  indicates  that  you
              passed  a weird option to curl that was passed on to libcurl and
              rejected. Read up in the manual!
 
       49     Malformed telnet option.
 
       51     The peer's SSL certificate or SSH MD5 fingerprint was not OK.
 
       52     The server didn't reply anything, which here  is	considered  an
              error.
 
       53     SSL crypto engine not found.
 
       54     Cannot set SSL crypto engine as default.
 
       55     Failed sending network data.
 
       56     Failure in receiving network data.
 
       58     Problem with the local certificate.
 
       59     Couldn't use specified SSL cipher.
 
       60     Peer  certificate cannot be authenticated with known CA certifi-
              cates.
 
       61     Unrecognized transfer encoding.
 
       62     Invalid LDAP URL.
 
       63     Maximum file size exceeded.
 
       64     Requested FTP SSL level failed.
 
       65     Sending the data requires a rewind that failed.
 
       66     Failed to initialise SSL Engine.
 
       67     The user name, password, or similar was not  accepted  and  curl
              failed to log in.
 
       68     File not found on TFTP server.
 
       69     Permission problem on TFTP server.
 
       70     Out of disk space on TFTP server.
 
       71     Illegal TFTP operation.
 
       72     Unknown TFTP transfer ID.
 
       73     File already exists (TFTP).
 
       74     No such user (TFTP).
 
       75     Character conversion failed.
 
       76     Character conversion functions required.
 
       77     Problem with reading the SSL CA cert (path? access rights?).
 
       78     The resource referenced in the URL does not exist.
 
       79     An unspecified error occurred during the SSH session.
 
       80     Failed to shut down the SSL connection.
 
       82     Could  not  load	CRL  file,  missing  or wrong format (added in
              7.19.0).
 
       83     Issuer check failed (added in 7.19.0).
 
       84     The FTP PRET command failed
 
       85     RTSP: mismatch of CSeq numbers
 
       86     RTSP: mismatch of Session Identifiers
 
       87     unable to parse FTP file list
 
       88     FTP chunk callback reported error
 
       89     No connection available, the session will be queued
 
       90     SSL public key does not matched pinned public key
 
       91     Invalid SSL certificate status.
 
       92     Stream error in HTTP/2 framing layer.