Report RpcException error thrown by _reloadSource vmservice rpc. (#16417)

* Don't return null, just rethrow RpcException error when vmservice responds with error.
diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index d2dd708..30f047a 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -755,7 +755,7 @@
     } on rpc.RpcException catch (error) {
       printError('Error ${error.code} received from application: ${error.message}');
       printTrace('${error.data}');
-      return null;
+      rethrow;
     }
   }