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
Yes, but in order to make the code more clear I am unifying the methods `is_valid_damage_ini` and `is_valid_risk_ini` into a single `is_valid_inputs`.