polKitAgent: Don't crash when we cannot enable/disable the auth agent

This makes it easier to run with jhbuild.

https://bugzilla.gnome.org/show_bug.cgi?id=687556

js/ui/components/polkitAgent.js

@@ -335,11 +335,19 @@ const AuthenticationAgent = new Lang.Class({
     },
 
     enable: function() {
+        try {
             this._native.register();
+        } catch(e) {
+            log('Failed to register AuthenticationAgent');
+        }
     },
 
     disable: function() {
+        try {
             this._native.unregister();
+        } catch(e) {
+            log('Failed to unregister AuthenticationAgent');
+        }
     },
 
     _onInitiate: function(nativeAgent, actionId, message, iconName, cookie, userNames) {