Only remove group service account from state in read if not found

This change improves error handling when reading a GitLab group service account. Previously, any error would cause the system to assume the service account was deleted and remove it from tracking. Now, the code is more selective - it only removes the service account from tracking when the error specifically indicates it wasn't found (contains "unable to find service account"). For all other types of errors, it properly reports the error instead of silently removing the resource. This prevents accidentally losing track of service accounts due to temporary network issues or other non-deletion related problems.

Fixes #6654 (closed)