Fix introduced potential TypeError with _get_command_error

discord/app_commands/errors.py

@@ -498,6 +498,7 @@ def _get_command_error(
                 else:
                     errors = {key: ' '.join(x.get('message', '') for x in inner_errors)}
 
+            if isinstance(errors, dict):
                 messages.extend(f'{indentation}  {k}: {v}' for k, v in errors.items())