pg_upgrade: Report full disk better

Previously, pg_upgrade would abort copy_file() on a short write without
setting errno, which the caller would report as an error with the
message "Success".  We assume ENOSPC in that case, as we do elsewhere in
the code.  Also set errno in some other error cases in copy_file() to
avoid bogus "Success" error messages.

This was broken in 6b711cf3, so 9.2 and
before are OK.