Add errors for various chooser failure modes, and remove assertions that are about to change.

LayoutTests/bluetooth/resources/bluetooth-helpers.js

 'use strict';
 
 // Sometimes we need to test that using either the name, alias, or UUID
 // produces the same result. The following objects help us do that.
 var generic_access = {
   alias: 0x1800,
   name: 'generic_access',
   uuid: '00001800-0000-1000-8000-00805f9b34fb'
 };
 var device_name = {
@@ skipping 62 matching lines @@
   return strAlias + '-97e5-4cd7-b9f1-f5a427670c59';
 }
 
 // Function to test that a promise rejects with the expected error type and
 // message.
 function assert_promise_rejects_with_message(promise, expected, description) {
   return promise.then(() => {
     assert_unreached('Promise should have rejected: ' + description);
   }, error => {
     assert_equals(error.name, expected.name, 'Unexpected Error Name:');
-    assert_equals(error.message, expected.message, 'Unexpected Error Message:');
+    if (expected.message) {
+      assert_equals(error.message, expected.message, 'Unexpected Error Message:');
+    }
   });
 }