internal/frontend: exit frontend fetch early on queue error

If enqueuing a frontend fetch task fails, log an error and return.

Change-Id: I360a7e8c6c83d341b29de1f332417047bb91fc02
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/356529
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/internal/frontend/fetch.go b/internal/frontend/fetch.go
index 92508fd..ec32d23 100644
--- a/internal/frontend/fetch.go
+++ b/internal/frontend/fetch.go
@@ -201,6 +201,9 @@
 			if _, err := s.queue.ScheduleFetch(ctx, modulePath, requestedVersion, opts); err != nil {
 				fr.err = err
 				fr.status = http.StatusInternalServerError
+				log.Errorf(ctx, "enqueuing %s@%s to frontend-fetch task queue: %v", modulePath, requestedVersion, err)
+				results[i] = fr
+				return
 			}
 			log.Debugf(ctx, "queued %s@%s to frontend-fetch task queue", modulePath, requestedVersion)