Optionally raise exception if lock was not successfully acquired

[pelletencate]

When a lock is not acquired, currently a semaphore returns false. When a lock has been successfully required, it returns the result of the passed through block.

If such block would result in false itself, right now, there is no way to tell if the lock was successfully acquired. In order to remedy this, we need a specific exception that can be raised in case a lock is not acquired.