Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

Additionally, in slotsync.c, replace one StringInfoData variable usage
with a constant string to avoid palloc/pfree. Also, replace the inclusion
of "logical.h" with "slot.h" to prevent the exposure of unnecessary
implementation details.

Reported-by: Kyotaro Horiguchi, Masahiko Sawada
Author: Shveta Malik based on suggestions by Robert Haas and Amit Kapila
Reviewed-by: Kyotaro Horiguchi, Amit Kapila
Discussion: https://postgr.es/m/20240214.162652.773291409747353211[email protected]
Discussion: https://postgr.es/m/20240219.134015.1888940527023074780[email protected]
Discussion: https://postgr.es/m/CAD21AoCYXhDYOQDAS-rhGasC2T+tYbV=8Y18o94sB=5AxcW+yA@mail.gmail.com