curl: add variables to --write-out

In particular, these ones can help a user to create its own error
message when one or transfers fail.

writeout: add 'onerror', 'url', 'urlnum', 'exitcode', 'errormsg'

onerror - lets a user only show the rest on non-zero exit codes

url - the input URL used for this transfer

urlnum - the numerical URL counter (0 indexed) for this transfer

exitcode - the numerical exit code for the transfer

errormsg - obvious

Reported-by: Earnestly on github
Fixes #6199
Closes #6207

docs/cmdline-opts/write-out.d

@@ -29,6 +29,12 @@ The variables available are:
 .B content_type
 The Content-Type of the requested document, if there was any.
 .TP
+.B errormsg
+The error message. (Added in 7.75.0)
+.TP
+.B exitcode
+The numerical exitcode. (Added in 7.75.0)
+.TP
 .B filename_effective
 The ultimate filename that curl writes out to. This is only meaningful if curl
 is told to write to a file with the --remote-name or --output
@@ -74,6 +80,10 @@ The number of response headers in the most recent request (restarted at each
 .B num_redirects
 Number of redirects that were followed in the request. (Added in 7.12.3)
 .TP
+.B onerror
+The rest of the output is only shown if the transfer returned a non-zero error
+(Added in 7.75.0)
+.TP
 .B proxy_ssl_verify_result
 The result of the HTTPS proxy's SSL peer certificate verification that was
 requested. 0 means the verification was successful. (Added in 7.52.0)
@@ -161,6 +171,12 @@ server needed to calculate the result.
 .B time_total
 The total time, in seconds, that the full operation lasted.
 .TP
+.B url
+The URL that was fetched. (Added in 7.75.0)
+.TP
+.B urlnum
+The URL index number of this transfer, 0-indexed. (Added in 7.75.0)
+.TP
 .B url_effective
 The URL that was fetched last. This is most meaningful if you've told curl
 to follow location: headers.