report fd/path if dump_one_reg_file()->lookup_nsid_by_mnt_id() fails

"Unable to look up the %d mount" doesn't really help to understand
the problem, add a bit more info.

And perhaps it makes more sense to change dump_task_files_seized()
to report fd/path if dump_one_file() fails.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>