Improve "user mapping not found" error message

Display the name of the foreign server for which the user mapping was
not found.

Author: Ian Lawrence Barwick <barwick@gmail.com>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAB8KJ=jFzNaeyFtLcTZNOc6fd1+F93pGVLFa-wyt31wn7VNxqQ@mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2717,10 +2717,10 @@ ALTER FOREIGN TABLE ft4 OPTIONS (ADD use_remote_estimate 'true');
 -- regress_view_owner_another, the view owner, though it fails as expected
 -- due to the lack of a user mapping for that user.
 EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;
-ERROR:  user mapping not found for "regress_view_owner_another"
+ERROR:  user mapping not found for user "regress_view_owner_another", server "loopback"
 -- Likewise, but with the query under an UNION ALL
 EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM (SELECT * FROM v4 UNION ALL SELECT * FROM v4);
-ERROR:  user mapping not found for "regress_view_owner_another"
+ERROR:  user mapping not found for user "regress_view_owner_another", server "loopback"
 -- Should not get that error once a user mapping is created
 CREATE USER MAPPING FOR regress_view_owner_another SERVER loopback OPTIONS (password_required 'false');
 EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;

src/backend/foreign/foreign.c

@@ -217,10 +217,14 @@ GetUserMapping(Oid userid, Oid serverid)
 	}
 
 	if (!HeapTupleIsValid(tp))
+	{
+		ForeignServer *server = GetForeignServer(serverid);
+
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("user mapping not found for \"%s\"",
+				 errmsg("user mapping not found for user \"%s\", server \"%s\"",
-						MappingUserName(userid))));
+						MappingUserName(userid), server->servername)));
+	}
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
 	um->umid = ((Form_pg_user_mapping) GETSTRUCT(tp))->oid;