Answer 499 to a client that goes away while waiting for an image (closes #339)

When a request waits for an image another request is making and its
client goes away, the image proxy returns context.Canceled. serveImage
now answers it 499, the status nginx calls "client closed request". It
logs it once at info level with the request_id, host and path. It was
answered 500 and logged at error level, as pixa's own failure. A
cancellation while the request's context is live is still answered 500.
The request making the image is still answered as before. README.md
lists the 499 on both image routes.

Model: opus-5-5

README.md

@@ -310,6 +310,8 @@ path under `/v1/` answers 200, in maintenance mode too.
     - 403 when the upstream host, or a host it redirects to, has an address in a
       blocked network (see `blocked_networks`).
     - 429 over `image_requests_per_minute` (below).
+    - 499 when the client goes away before its image is ready, unless its
+      request is the one making the image (below).
     - 415 when the upstream answers with a `Content-Type` that is not one of the
       source formats (below).
     - 415 when the upstream sends a body whose first bytes are not those of the
@@ -341,6 +343,7 @@ path under `/v1/` answers 200, in maintenance mode too.
         - 403
         - 415
         - 429
+        - 499
         - 502
         - 503
         - 504