pgsql: Fix inconsistent elevel in pg_sync_replication_slots() retry log

Fix inconsistent elevel in pg_sync_replication_slots() retry logic.

The commit 0d2d4a0ec3 allowed pg_sync_replication_slots() to retry sync
attempts, but missed a case, when WAL prior to a slot's
confirmed_flush_lsn is not yet flushed locally.

By changing the elevel from ERROR to LOG, we allow the sync loop to
continue. This provides the opportunity for the slot to be synchronized
once the standby catches up with the necessary WAL.

Author: Zhijie Hou <houzj(dot)fnst(at)fujitsu(dot)com>
Reviewed-by: shveta malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAFPTHDZAA+gWDntpa5ucqKKba41=tXmoXqN3q4rpjO9cdxgQrw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f1ddaa15357fccb03dea371f8cb481ce2c5e4808

Modified Files
--------------
doc/src/sgml/func/func-admin.sgml | 4 +++-
src/backend/replication/logical/slotsync.c | 6 +++++-
2 files changed, 8 insertions(+), 2 deletions(-)