Only build the default error message if the literal was invalid

Author: obrie

lib/graphql/static_validation/rules/variable_default_values_are_correctly_typed.rb

@@ -27,9 +27,8 @@ def validate_default_value(node, context)
               error_message = err.message
             end
 
-            error_message ||= "Default value for $#{node.name} doesn't match type #{type}"
-
             if !valid
+              error_message ||= "Default value for $#{node.name} doesn't match type #{type}"
               context.errors << message(error_message, node, context: context)
             end
           end