common.js: show error on ajax failures.

Make `link_to ..., remote: true` ajax requests display an error notice on failure.

app/assets/javascripts/common.js

@@ -39,6 +39,11 @@ $(function() {
       location.reload();
     });
   });
+
+  // triggered by rails when a `link_to ..., remote: true` call fails.
+  $(document).on("ajax:error", function (event, xhr, status, error) {
+    Danbooru.error("Error: " + xhr.status + " " + xhr.statusText);
+  });
 });
 
 var Danbooru = {};