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