pgsql: Check slot->restart_lsn validity in a few more places

Check slot->restart_lsn validity in a few more places

Lack of these checks could cause visible misbehavior, including
assertion failures.  This was missed in commit c6550776394e, whereby
restart_lsn becomes invalid when the size limit is exceeded.

Also reword some existing error messages, and add errdetail(), so that
the reported errors all match in spirit.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200408.093710.447591748588426656.horikyota.ntt@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d0abe78d84274cc203f3d117b8006dc2164ca31a

Modified Files
--------------
contrib/test_decoding/expected/slot.out        |  3 ++-
src/backend/replication/logical/logicalfuncs.c | 13 +++++++++++++
src/backend/replication/slotfuncs.c            |  4 +++-
src/backend/replication/walsender.c            | 14 +++++++++++++-
4 files changed, 31 insertions(+), 3 deletions(-)