Improve error message annotation for GRANT/REVOKE on untrusted PLs.

Improve error message annotation for GRANT/REVOKE on untrusted PLs.

The annotation for "ERROR: language "foo" is not trusted" used to say
"HINT: Only superusers can use untrusted languages", which was fairly
poorly thought out.  For one thing, it's not a hint about what to do,
but a statement of fact, which makes it errdetail.  But also, this
fails to clarify things much, because there's a missing step in the
chain of reasoning.  I think it's more useful to say "GRANT and REVOKE
are not allowed on untrusted languages, because only superusers can use
untrusted languages".

It's been like this for a long time, but given the lack of previous
complaints, I don't think this is worth back-patching.

Discussion: <1417.1466289901@sss.pgh.pa.us>