Unclear HTTP-TPC error messages

• moving from gridftp to webdav decreased clarity of error messages
  • generic HTTP 500 error not very useful to diagnose problems
  • CMS suggest to create project with aim to improve situation
    • followup discussion in the WLCG DOMA TPC mailing list
      • account out of quota or filesystem out of space
      • missing/no such directory/file
      • file or server-with-file unavailable
      • permission creating directory/file
      • certificate/token expired
      • other authorization error
      • transfer timeout
      • connection reset/interrupt
      • service on other endpoint unreachable <IP address (or name, if no DNS entry)>
      • number of transfers/connections/requests exceeeded
      • canceled/aborted transfer
• response from FTS developers
  • error messages try to signalize where they comes from (storage / gfal / FTS)
  • storage must report reasonable responses in the first place
• extend compliance testsuite
  • e.g. same HTTP return codes as a response for given storage issue
• details about unclear XRootD/HTTP error messages should be reported via github issue