Make error message in Dart Debug Extension more specific (#1583)

Author: elliette

dwds/debug_extension/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 1.29
 
 - Notify the debugger and inspector panels when the debug session is disconnected.
+- Provide a detailed error message when the debugger fails to connect.
 
 ## 1.28
 

dwds/debug_extension/web/background.dart

@@ -23,9 +23,10 @@ import 'package:sse/client/sse_client.dart';
 import 'package:web_socket_channel/web_socket_channel.dart';
 
 const _notADartAppAlert = 'No Dart application detected.'
-    ' Your development server should inject metadata to indicate support for'
-    ' Dart debugging. This may require setting a flag. Check the documentation'
-    ' for your development server.';
+    ' Are you trying to debug an application that includes a Chrome hosted app'
+    ' (an application listed in chrome://apps)? If so, debugging is disabled.'
+    ' You can fix this by removing the application from chrome://apps. Please'
+    ' see https://bugs.chromium.org/p/chromium/issues/detail?id=885025#c11.';
 
 // Extensions allowed for cross-extension communication.
 const _allowedExtensions = {