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

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