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

* Don't return null, just rethrow RpcException error when vmservice responds with error.

packages/flutter_tools/lib/src/run_hot.dart

@@ -534,7 +534,9 @@ class HotRunner extends ResidentRunner {
           pause: pause
         );
         countExpectedReports += reports.length;
-        Future.wait(reports).then((List<Map<String, dynamic>> list) {
+        Future.wait(reports).catchError((dynamic error) {
+          return <Map<String, dynamic>>[error];
+        }).then((List<Map<String, dynamic>> list) {
           // TODO(aam): Investigate why we are validating only first reload report,
           // which seems to be current behavior
           final Map<String, dynamic> firstReport = list.first;

packages/flutter_tools/lib/src/vmservice.dart

@@ -755,7 +755,7 @@ class VM extends ServiceObjectOwner {
     } on rpc.RpcException catch (error) {
       printError('Error ${error.code} received from application: ${error.message}');
       printTrace('${error.data}');
-      return null;
+      rethrow;
     }
   }