pgsql: Better error messages for short reads/writes in SLRU

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ef121647f7f29b59b86eca3dee9b90548245a3a

Modified Files
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src/backend/access/transam/slru.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)