Throw an exception if a condition is missing

When running tests defining conditions in compact XML I realized that if there's a typo in the condition name we get an error message, but the application is allowed to continue.

If the condition declared in compact/trunk/MyDet/conditions.xml is used in the detector constructor (e.g. in something like MyDetObject constructor) we get an exception much earlier than this point, but sometimes we just want to declare a condition without using it right away.