When certain event-trigger-only functions are called when not in the
wrong context, they were reporting the "feature not supported" SQLSTATE,
which is somewhat misleading.  Create a new custom error code for such
uses instead.

Not backpatched since it may be seen as an undesirable behavioral
change.

Author: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAB7nPqQ-5NAkHQHh_NOm7FPep37NCiLKwPoJ2Yxb8TDoGgbYYA@mail.gmail.com