extensionSystem: Use logError to record extension errors with stack trace

Extensions might emit JS errors explicitly or implicitly, however GNOME
Shell doesn't present any stack trace for those making them quite hard
to debug.

Make this easier by logging errors with logError() whichs includes the
stack dump.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/700

js/ui/extensionSystem.js

@@ -194,7 +194,7 @@ var ExtensionManager = class {
             extension.errors = [];
         extension.errors.push(message);
 
-        log('Extension "%s" had error: %s'.format(uuid, message));
+        logError(error, `Extension ${uuid}`);
         this.emit('extension-state-changed', extension);
     }