pmb.netboot: Only attempt to install nbd after rootfs existance check + Use NonBugError for nonexistent rootfs code path

See individual commits. Together, these change the output in the case of a nonexistent rootfs from

$ pmb netboot serve
[11:54:29] (native) install nbd
[11:54:29] (native) chroot not initialized! This is a bug! Please report it.
[11:54:29] (native) initializing the chroot for you...
[sudo] password for newbyte: 
[11:54:35] Download http://dl-cdn.alpinelinux.org/alpine/edge/main/x86_64/apk-tools-static-2.14.9-r0.apk
[11:54:35] (native) Creating chroot
[11:54:37] ERROR: The rootfs has not been generated yet, please run 'pmbootstrap install' first.
[11:54:37] See also: <https://postmarketos.org/troubleshooting>

Run 'pmbootstrap log' for details.

Before you report this error, ensure that pmbootstrap is up to date.
Find the latest version here: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/tags
Your version: 3.2.0
Channel: systemd-edge (pmaports: master)
Device:  qemu-amd64 (x86_64, kernel: lts)
UI:      phosh
systemd: yes (default for selected UI)

to

$ pmb netboot serve
[11:59:11] ERROR: The rootfs has not been generated yet, please run 'pmbootstrap install' first.

which I think is much better.