pgsql: Improve a couple of error messages.

Improve a couple of error messages.

Change "function" to "function or procedure" in
PreventInTransactionBlock, and improve grammar of ExecWaitStmt's
complaint about having an active snapshot.

Author: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Reviewed-by: Andreas Karlsson <andreas(at)proxel(dot)se>
Reviewed-by: Marcos Pegoraro <marcos(at)f10(dot)com(dot)br>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAFj8pRCveWPR06bbad9GnMb0Kcr6jnXPttv9XOaOB+oFCD1Tsg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/54315fde73915a2468eaa22a2fa38927a23ac4ec

Modified Files
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src/backend/access/transam/xact.c | 3 ++-
src/backend/commands/wait.c | 4 ++--
src/test/recovery/t/049_wait_for_lsn.pl | 4 ++--
src/test/regress/expected/subscription.out | 2 +-
4 files changed, 7 insertions(+), 6 deletions(-)