HTTP Compliance: reply with 504 (Gateway Timeout) if required validation fails.

HTTP Compliance: reply with 504 (Gateway Timeout) if required validation fails.

RFC 2616 says that we MUST reply with 504 (Gateway Timeout) if validation
fails and cached reply has proxy-revalidate, must-revalidate or s-maxage
Cache-Control directive.

FwdState::makeConnectingError() method is added to set error status depending
on whether the request was a validation request.

Co-Advisor test cases:
    test_case/rfc2616/noSrv-hit-must-reval-s-maxage-resp
    test_case/rfc2616/noSrv-hit-must-reval-proxy-revalidate-resp
    test_case/rfc2616/noSrv-hit-must-reval-must-revalidate-resp