Fixed a TypeError with join in __init__ for 422 responses

On your latest revision with Chiliproject 3.8.0 I got the following exception when creating an issue with a parent outside of the selected project:

    File "redmine/__init__.py", line 132, in request
        raise ValidationError(to_string(', '.join(json_response(response.json)['errors'])))
    TypeError: sequence item 0: expected string, list found

I fixed this problem for my case by joining twice if not all errors are strings.

Now it raises a ValidationError with the following message:

    parent_issue_id: doesn't belong to the same project