BTBSession fails to select a proper Tunable when normalized_scores becomes None

[pvk-developer]

• BTB version: 0.3.5
• Python version: Python 3.6
• Operating System: Ubuntu 18.04

Description

When we are runing BTBSession and there is a list of Tunables to try, once the max error is reached and self._normalized_scores becomes None, the BTBSession fails to select the proper Tunable.
This happens because we are doing a selection over the self._tuners list which is not updated after a Tunable is removed because of reaching max errors. In this case we have to use self._tunables in order to select from which one to continue tuning with. And if there is a case that this list is empty, then raise a StopTuning exception.