pg_upgrade: Make format strings consistent

pg_upgrade: Make format strings consistent

The backport of commit f295494d338 introduced a format string using
%m.  This is not wrong, since those have been supported since commit
d6c55de1f99a, but only commit 2c8118ee5d9 later introduced their use
in this file.  This use introduces a gratuitously different
translatable string and also makes it inconsistent with the rest of
the file.  To avoid that, switch this back to the old-style strerror()
route in the appropriate backbranches