The image routes answered 500 and logged at error level when the upstream
host had no address, when the fetch failed before the upstream answered,
such as on a refused connection, or when the fetcher refused a redirect.
The fetcher now marks such a failure with ErrUpstreamRequestFailed; a
cancelled fetch is not marked. respondImageError answers it, and
ErrInvalidHost, 502 "upstream error" and logs them at warn level. A
timeout that does not match context.DeadlineExceeded, such as the TLS
handshake timeout, is now ErrUpstreamTimeout and answers 504. README
Routes lists these answers.
Model: opus-5-5
