New error code to track unsupported contexts

Hi all,

When pg_event_trigger_dropped_objects is run in a context that is not
the one of an event trigger, currently the error code
ERRCODE_FEATURE_NOT_SUPPORTED is returned. Wouldn't it be better to
have an error to define an out-of-context instead? It seems that it
would be a good thing to have more error verbosity for situations like
the case above. Note that this idea has been mentioned on this ML a
couple of weeks back. In any case, attached is a patch showing the
idea.

Opinions? Is that worth having?
Regards,
-- 
Michael