Remove incorrect toString() for URL in RestClientGraphQLClient error handling

[paulbakker]

Fix for #2059.

By default, the error handling for failing HTTP requests is done by RestClient itself, so the extra error check we have in code is typically not used. Added a test for this.

If a non-default status code handler is configured on the RestClient, it might still fall through, so I kept the existing error handling. However, I removed the incorrect toString() use for the url field in the exception.
In a RestClient you can't actually see what URL it is configured for, so we don't have access to the URL during error handling in this case. This is acceptable because this exception only throws when the user overrides the handling, and not handle a status code correctly.