Report update errors or timeouts.

Looking through Sentry history, `update <domain> err:` is an extremely
high SNR signal of something going wrong; from configuration errors on
our side, to people pushing too-large git repositories and it failing.
Either way we should know.

src/observe.go

@@ -130,6 +130,12 @@ func FiniObservability() {
 	}
 }
 
+func ObserveError(err error) {
+	if hasSentry() {
+		go sentry.CaptureException(err)
+	}
+}
+
 type observedResponseWriter struct {
 	inner  http.ResponseWriter
 	status int

src/pages.go

@@ -328,6 +328,7 @@ func putPage(w http.ResponseWriter, r *http.Request) error {
 
 	switch result.outcome {
 	case UpdateError:
+		ObserveError(result.err)
 		if errors.Is(result.err, ErrManifestTooLarge) {
 			w.WriteHeader(http.StatusRequestEntityTooLarge)
 		} else if errors.Is(result.err, errArchiveFormat) {
@@ -338,6 +339,7 @@ func putPage(w http.ResponseWriter, r *http.Request) error {
 			w.WriteHeader(http.StatusServiceUnavailable)
 		}
 	case UpdateTimeout:
+		ObserveError(fmt.Errorf("update timeout"))
 		w.WriteHeader(http.StatusGatewayTimeout)
 	case UpdateNoChange:
 		w.Header().Add("X-Pages-Update", "no-change")
@@ -511,9 +513,11 @@ func postPage(w http.ResponseWriter, r *http.Request) error {
 
 	switch result.outcome {
 	case UpdateError:
+		ObserveError(result.err)
 		w.WriteHeader(http.StatusServiceUnavailable)
 		fmt.Fprintf(w, "update error: %s\n", result.err)
 	case UpdateTimeout:
+		ObserveError(fmt.Errorf("update timeout"))
 		w.WriteHeader(http.StatusGatewayTimeout)
 		fmt.Fprintln(w, "update timeout")
 	case UpdateNoChange: