pgsql: Message corrections for partition split/merge commands

Message corrections for partition split/merge commands

Fix spelling and grammar, turn an accidental duplicate errmsg into
errdetail, and remove an errposition that was not pointing at anything
relevant to the error.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Jian He <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Yuchen Li <liyuchen_xyz(at)163(dot)com> (earlier version)
Discussion: https://postgr.es/m/CAJTYsWUvMT5uKOasPnm6-o9CrdXbRONiAYHTKJb7wx66LB8S1A@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/52e629be95fc2ff52eabdae5b72c8eb6d8eef38d

Modified Files
--------------
src/backend/commands/tablecmds.c | 2 +-
src/backend/parser/parse_utilcmd.c | 25 ++++++++--------
src/backend/partitioning/partbounds.c | 16 +++++------
src/test/regress/expected/partition_merge.out | 20 ++++++-------
src/test/regress/expected/partition_split.out | 41 +++++++++++++--------------
5 files changed, 51 insertions(+), 53 deletions(-)