handle failures to set up the debug service (#309)

Author: jakemac53

webdev/lib/src/serve/handlers/dev_handler.dart

@@ -108,8 +108,20 @@ class DevHandler {
           return;
         }
 
-        var appServices =
-            await loadAppServices(message.appId, message.instanceId);
+        AppDebugServices appServices;
+        try {
+          appServices =
+              await loadAppServices(message.appId, message.instanceId);
+        } catch (_) {
+          connection.sink.add(
+              jsonEncode(webdev.serializers.serialize(DevToolsResponse((b) => b
+                ..success = false
+                ..error = 'Webdev was unable to connect debug services to your '
+                    'application. Most likely this means you are trying to '
+                    'load in a different Chrome window than was launched by '
+                    'webdev.'))));
+          return;
+        }
 
         // Check if we are already running debug services for a different
         // instance of this app.