MM-10863: Handle non-API errors with redirect to webapp (#8943)

* MM-10863: Handle non-API errors with redirect to webapp

* Properly shutdown the app in the new test

web/handlers.go

@@ -157,8 +157,12 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			c.Err.IsOAuth = false
 		}
 
-		w.WriteHeader(c.Err.StatusCode)
-		w.Write([]byte(c.Err.ToJson()))
+		if IsApiCall(r) || len(r.Header.Get("X-Mobile-App")) > 0 {
+			w.WriteHeader(c.Err.StatusCode)
+			w.Write([]byte(c.Err.ToJson()))
+		} else {
+			utils.RenderWebAppError(w, r, c.Err, c.App.AsymmetricSigningKey())
+		}
 
 		if c.App.Metrics != nil {
 			c.App.Metrics.IncrementHttpError()