pgsql: Improve "user mapping not found" error message

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/7e5f5177996150bf3845562d00d41d1e1847d7e3

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 4 ++--
src/backend/foreign/foreign.c                  | 8 ++++++--
2 files changed, 8 insertions(+), 4 deletions(-)