Fix some BufFileRead() error reporting

Fix some BufFileRead() error reporting

Remove "%m" from error messages where errno would be bogus.  Add short
read byte counts where appropriate.

This is equivalent to what was done in
7897e3bb902c557412645b82120f4d95f7474906, but some code was apparently
developed concurrently to that and not updated accordingly.

Reviewed-by: Amit Kapila 
Discussion: https://www.postgresql.org/message-id/flat/f3501945-c591-8cc3-5ef0-b72a2e0eaa9c@enterprisedb.com