Improve error message on error

[jmini]

When the returned exception is this:

2 - Received server response on thread executor-thread-8
2 < 400
2 < Cache-Control: no-cache
2 < Content-Length: 99
2 < Content-Type: application/json
2 < Date: Mon, 31 Mar 2025 09:06:54 GMT
2 < Server: nginx
2 < Strict-Transport-Security: max-age=63072000
2 < Vary: Origin
2 < X-Content-Type-Options: nosniff
2 < X-Frame-Options: SAMEORIGIN
2 < X-Gitlab-Meta: {"correlation_id":"01JQNQ8AP39PMJ739E5N61PKWX","version":"1"}
2 < X-Request-Id: 01JQNQ8AP39PMJ739E5N61PKWX
2 < X-Runtime: 0.730485
{"message":{"project_namespace.name":["has already been taken"],"name":["has already been taken"]}}

The current error message is:

org.gitlab4j.api.GitLabApiException: The following fields have validation errors: project_namespace.name, name

After this merge request, the details about the errors names are provided as well:

org.gitlab4j.api.GitLabApiException: The following fields have validation errors: project_namespace.name, name
* project_namespace.name
     - has already been taken
* name
     - has already been taken