libpq: Align oauth_json_set_error() with other NLS patterns

libpq: Align oauth_json_set_error() with other NLS patterns

Now that the prior commits have fixed missing OAuth translations, pull
the bespoke usage of libpq_gettext() for OAUTHBEARER parsing into
oauth_json_set_error() itself, and make that a gettext trigger as well,
to better match what the other sites are doing. Add an _internal()
variant to handle the existing untranslated case.

Suggested-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/0EEBCAA8-A5AC-4E3B-BABA-0BA7A08C361B%40yesql.se
Backpatch-through: 18