extensionSystem: Save extension errors per-extension

Extension developers may be confused about why their extensions aren't working:
the LookingGlass isn't a very obvious place, or even which errors are theirs.
To remedy this, save all errors per-UUID which allows them to be retrieved
later.

https://bugzilla.gnome.org/show_bug.cgi?id=654770

js/ui/shellDBus.js

@@ -20,6 +20,10 @@ const GnomeShellIface = {
                 inSignature: 's',
                 outSignature: 'a{sv}'
               },
+              { name: 'GetExtensionErrors',
+                inSignature: 's',
+                outSignature: 'as'
+              }
              ],
     signals: [],
     properties: [{ name: 'OverviewActive',
@@ -80,6 +84,10 @@ GnomeShell.prototype = {
         return ExtensionSystem.extensionMeta[uuid] || {};
     },
 
+    GetExtensionErrors: function(uuid) {
+        return ExtensionSystem.errors[uuid] || [];
+    },
+
     get OverviewActive() {
         return Main.overview.visible;
     },