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

LayoutTests/bluetooth/resources/bluetooth-helpers.js

Index: LayoutTests/bluetooth/resources/bluetooth-helpers.js
diff --git a/LayoutTests/bluetooth/resources/bluetooth-helpers.js b/LayoutTests/bluetooth/resources/bluetooth-helpers.js
index a5938a7d889a321abc068052341c1bdf2a175106..064880493c829a79d49516cd89c63e9b8beff53e 100644
--- a/LayoutTests/bluetooth/resources/bluetooth-helpers.js
+++ b/LayoutTests/bluetooth/resources/bluetooth-helpers.js
@@ -80,6 +80,8 @@ function assert_promise_rejects_with_message(promise, expected, description) {
     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:');
+    }
   });
 }