Return message when personal access token creation fails in internal API
This is a follow-up to the discussion in gitlab-shell!397 (merged) to move expiry date validation onto the API side. See:
gitlab-shell!397 (comment 391081491))
Before this change the internal api would return a 500 error to the user when creation of the personal access token fails, for example, when a maximum token lifetime is enforced.
After this change, the internal api will return a more meaningful error message to the user.