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,
or when the fetcher refused a redirect. Such a failure is now
ErrInvalidHost or ErrUpstreamRequestFailed, which respondImageError
answers 502 "upstream error" and logs at warn level. A TLS handshake, an
address lookup or a connection that times out is now ErrUpstreamTimeout
and answers 504; its log line keeps the cause. A cancelled fetch or
address lookup is neither ErrUpstreamRequestFailed nor ErrInvalidHost.
README Routes lists these answers.
Model: opus-5-5
