Improve the error message when the user sets the wrong calculation_mode

See https://bugs.launchpad.net/oq-engine/+bug/1385088. The tests are green: https://ci.openquake.org/job/zdevel_oq-risklib/98
Is it desired that `is_valid_damage_ini` returns `True` in case `'damage' not in self.calculation_mode`?