cli: Change `switch` to give more useful error/info on non-bootc system

[vrothberg]

openat(AT_FDCWD, "/run/ostree-booted", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

When running bootc switch on a non-ostree system it errors out with

ERROR Switching: Acquiring sysroot: No such file or directory

I think bootc can help the user a bit more and maybe guide them into installing the image to disk rather than switching.

Cc: @cgwalters