Provide better error diagnostic when GitLab project path is incorrect

If the GitLab namespace element is specified incorrectly in a spacelift_stack Terraform resource, the stack execution will fail with the confusing error message “internal error”. It would be preferable if an error message stating that the specified project cannot be found in the specified namespace. This error was encountered after refactoring some Terraform code wherein it was re-organized into GitLab subgroups.

Note that in contrast to specifying an incorrect namespace, no error is flagged if the project_root is specified incorrectly, however, the requested stack is not actually created. This situation should also produce a useful error / diagnostic message identifying the underlying issue.