Improve usability of base.Scenario.validate()

By default Validator() shouldn't raise any exception, cause it is
not catch by validate method. To make validate more safe we should
catch any exception from validators and convert them to
InvalidScenarioArgument exception.

Change-Id: Ib685617ccef1cc62e0e7a49bb5392e6fe44f6a55
Merge "Improve usability of base.Scenario.validate()"