Remove a couple of strerror() calls

Remove a couple of strerror() calls

Change to using %m in the error message string.  We need to be a bit
careful here to preserve errno until we need to print it.

This change avoids the use of not-thread-safe strerror() and unifies
some error message strings, and maybe makes the code appear more
consistent.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/daa87d79-c044-46c4-8458-8d77241ed7b0%40eisentraut.org