setup_devices: fall back to setup_devices_bind if setup_devices_simple failed irrespective of the container

This allows using debootstrap in unshared user namespaces without having to set container=lxc, which is a lie and just a hack to make debootstrap bind-mount the device nodes instead of using mknod.

This change was suggested by @smcv here: !99 (comment 423138)

This came up again as now @helmutg is working on making piuparts work unprivileged which currently requires setting container=lxc.

I'm sneaking in one unrelated commit that fixes a misleading error message when the user forgets to put /sbin in their PATH as then debootstrap will quit with E: Unable to execute target architecture which is wrong and thus confusing.