Answer 502 for an upstream host with no address or a failed fetch (closes #367)

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

README.md

@@ -337,8 +337,15 @@ to any method the list does not give for it.
       neither the image nor its source in its cache.
     - 502 likewise for 5 minutes after a body whose first bytes are not those of
       its `Content-Type`.
+    - 502 when the upstream host, or a host it redirects to, has no address.
+    - 502 when the upstream redirects to a URL that is not `https`, or redirects
+      10 times.
+    - 502 when the fetch fails before the upstream answers, such as on a refused
+      connection or a failed TLS handshake.
     - 503 when pixa is busy (see Limits).
     - 503 in maintenance mode.
+    - 504 when the upstream has not finished the TLS handshake within 10
+      seconds.
     - 504 when the upstream has not sent its response headers within
       `upstream_fetch_timeout`.
     - 500 when that time runs out while the image itself is still arriving.