Better error messages for short reads/writes in SLRU
This avoids getting a
Could not read from file ...: Success.
for a short read or write (since errno is not set in that case).
Instead, report a more specific error messages.
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/
5de61b6b-8be9-7771-0048-
860328efe027%402ndquadrant.com