Make more consistent some error messages for file-related operations

Make more consistent some error messages for file-related operations

Some error messages which report something about a file operation use
as well context which is already provided within the path being worked
on, making things rather duplicated.  This creates more work for
translators, and does not actually bring clarity.

More could be done, however in a lot of cases the context used is
actually useful, still that patch gets down things with a good cut.

Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi, Tom Lane
Discussion: https://postgr.es/m/20180718044711.GA8565@paquier.xyz