authPrompt: Do not enable sensitivity if retries are disallowed

Set the sensitivity of the UI according to the canRetry parameter and thus
if no more logins are allowed don't take any input.

Fixes #311

js/gdm/authPrompt.js

@@ -242,11 +242,11 @@ var AuthPrompt = new Lang.Class({
         this.emit('prompted');
     },
 
-    _onVerificationFailed() {
+    _onVerificationFailed(userVerifier, canRetry) {
         this._queryingService = null;
         this.clear();
 
-        this.updateSensitivity(true);
+        this.updateSensitivity(canRetry);
         this.setActorInDefaultButtonWell(null);
         this.verificationStatus = AuthPromptStatus.VERIFICATION_FAILED;
     },

js/gdm/util.js

@@ -565,7 +565,7 @@ var ShellUserVerifier = new Lang.Class({
             }
         }
 
-        this.emit('verification-failed');
+        this.emit('verification-failed', canRetry);
     },
 
     _onConversationStopped(client, serviceName) {