You can change the appropriate system text, which is indeed 'Username / Password incorrect'. If you replace that text with one explaining the failed login could be due to providing incorrect credentials or due to the licensepool being full in terms of concurrent users, it should be a bit clearer than the default option.
Unfortunately we don't currently have a way to distinguish between a failed login due to the concurrent user limit of the licensepool (which is done by throwing an exception in the login process if this happens) and a user just providing a wrong password.