MD Settings: Display error messages in "add" and "edit" exception dialogs.

chrome/browser/resources/settings/site_settings/edit_exception_dialog.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview 'settings-edit-exception-dialog' is a component for editing a
  * site exception entry.
  */
 Polymer({
   is: 'settings-edit-exception-dialog',
 
   properties: {
     /**
      * @type {!SiteException}
      */
     model: Object,
 
     /** @private */
     origin_: String,
+
+    /**
+     * Whether the current input is invalid.
+     * @private
+     */
+    invalid_: {
+      type: Boolean,
+      value: false,
+    },
   },
 
   /** @private {!settings.SiteSettingsPrefsBrowserProxy} */
   browserProxy_: null,
 
   /** @override */
   attached: function() {
     this.browserProxy_ =
         settings.SiteSettingsPrefsBrowserProxyImpl.getInstance();
     this.origin_ = this.model.origin;
@@ skipping 31 matching lines @@
    * @param {!KeyboardEvent} e
    * @private
    */
   onKeypress_: function(e) {
     if (e.key == 'Enter' && !this.$.actionButton.disabled)
       this.onActionButtonTap_();
   },
 
   /** @private */
   validate_: function() {
+    if (this.$$('paper-input').value.trim() == '') {
+      this.invalid_ = true;
+      return;
+    }
+
     this.browserProxy_.isPatternValid(this.origin_).then(function(isValid) {
-      this.$.actionButton.disabled = !isValid;
+      this.invalid_ = !isValid;
     }.bind(this));
   },
 });