patch: improve "user mapping not found" error message

Hi

Mild corner-case annoyance while doing Random Experimental Things:

postgres=# SELECT * FROM parttest;
ERROR: user mapping not found for "postgres"

Okaaaay, but which server?

postgres=# \det
List of foreign tables
Schema | Table | Server
--------+---------------+-----------
public | parttest_10_1 | fdw_node2
public | parttest_10_3 | fdw_node3
public | parttest_10_5 | fdw_node4
public | parttest_10_7 | fdw_node5
public | parttest_10_9 | fdw_node6
(5 rows)

(Muffled sound of small patch hatching) aha:

postgres=# SELECT * FROM parttest;
ERROR: user mapping not found for user "postgres", server "fdw_node5"

Regards

Ian Barwick