Fix error messages.

generator/plugins/proto2gql/services.go

@@ -203,7 +203,7 @@ func (g Proto2GraphQL) serviceMethod(sc ServiceConfig, cfg MethodConfig, file *p
 
 	if len(method.OutputMessage.Fields) == 1 && !cfg.UnwrapResponseField {
 		fmt.Printf(
-			"Suggestion: service `%s` method `%s` in file `%s` has 1 output argument. Can be unwrapped.\n",
+			"Suggestion: service `%s` method `%s` in file `%s` has 1 output field. Can be unwrapped.\n",
 			method.Service.Name,
 			method.Name,
 			file.File.FilePath,
@@ -261,7 +261,7 @@ func (g Proto2GraphQL) addDataLoaderProvider(sc ServiceConfig, cfg MethodConfig,
 		_, ok := method.OutputMessage.Fields[0].Type.(*parser.Message)
 
 		if !ok {
-			return errors.New("OutputMessage field must be message")
+			return errors.Errorf("can't unwrap %s method. Response must be message", method.Name)
 		}
 
 		outProtoType = method.OutputMessage.Fields[0].Type.(*parser.Message).Fields[0]