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
This is on purpose. It is true that in the dictionary the key is 'fragility', but in the configuration file (the one the user sees) that variable is called 'fragility_file'. The suffix `_file` is stripped internally but the user does not need to know that.