PLCgen: forbid events without controllability
While it likely worked since events are not actually used, it seems conceptually wrong to allow events without controllability in PLCgen.
With !857 (merged) (see #628), adding support for such events will be even more complicated, as that patch splits events between controllable and uncontrollable events. In that setup, there is no logical spot for events without controllability any more.
The solution therefore seems to forbid such events in the PLCgen pre-checker.