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>

Author: tglsfdc

src/backend/catalog/aclchk.c

@@ -2592,7 +2592,8 @@ ExecGrant_Language(InternalGrant *istmt)
 					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 					 errmsg("language \"%s\" is not trusted",
 							NameStr(pg_language_tuple->lanname)),
-				   errhint("Only superusers can use untrusted languages.")));
+					 errdetail("GRANT and REVOKE are not allowed on untrusted languages, "
+				   "because only superusers can use untrusted languages.")));
 
 		/*
 		 * Get owner ID and working copy of existing ACL. If there's no ACL,

src/test/regress/expected/privileges.out

@@ -536,7 +536,7 @@ REVOKE ALL PRIVILEGES ON LANGUAGE sql FROM PUBLIC;
 GRANT USAGE ON LANGUAGE sql TO regressuser1; -- ok
 GRANT USAGE ON LANGUAGE c TO PUBLIC; -- fail
 ERROR:  language "c" is not trusted
-HINT:  Only superusers can use untrusted languages.
+DETAIL:  GRANT and REVOKE are not allowed on untrusted languages, because only superusers can use untrusted languages.
 SET SESSION AUTHORIZATION regressuser1;
 GRANT USAGE ON LANGUAGE sql TO regressuser2; -- fail
 WARNING:  no privileges were granted for "sql"