Emitting the plain error number is not very helpful. Use strerror()
instead.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160615153630.2116-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>