Specific error code / message for federated download timeouts

[Half-Shot]

Currently if federated media takes too long to download, the media repo responds with Replying with result: *_responses.ErrorResponse &{Code:M_UNKNOWN Message:Unexpected Error InternalCode:M_UNKNOWN}, which somewhat masks the true cause which is that upstream is slow. This would aid debugging issues where users report 500 errors on media, which often are the result of other homeserver issues.