Missing rsync binary leads to assertion

[phmccarty]

In swupd_create_update, an assertion is raised when the rsync binary is not installed. It would be more helpful if swupd_create_update exited gracefully with an error for this condition and report to the user "rsync not installed; cannot continue", or similar.

[phmccarty]

Also, the full directory is already created by that point, so that directory should be removed before exiting, else the directory's existence will make swupd_create_update skip the full chroot creation step, leading to the generation of corrupt content.