Use the appropriate HTTP response code for backend timeouts

[robstradling]

Upon timeout or cancellation of a backend connection context, CTFE currently returns HTTP 408 to the client. This seems wrong, because 408 is intended to mean that the client did not send its request quickly enough.

HTTP 504 (Gateway Timeout) seems a more appropriate error code, given that the timeout or cancellation occurred because an "upstream server" (i.e., the Trillian backend) did not respond quickly enough:

The 504 (Gateway Timeout) status code indicates that the server,
while acting as a gateway or proxy, did not receive a timely response
from an upstream server it needed to access in order to complete the
request.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.

[mhutchinson]

/gcbrun

[mhutchinson]

/gcbrun

[mhutchinson]

Thanks!