install: improve error message when systemd-boot is selected without composefs backend

[andrewdunndev]

Summary

When --bootloader=systemd is specified without --composefs-backend, the error message is:

error: Installing to disk: bootupd is required for ostree-based installs

This doesn't tell the user what to do. The fix is to use --composefs-backend, but the error message doesn't mention composefs at all. It reads like bootupd is a hard requirement, when actually systemd-boot works fine with the composefs backend.

Reproduction

podman run --rm --privileged --pid=host \
    -v /dev:/dev -v /var/lib/containers:/var/lib/containers \
    quay.io/fedora/fedora-bootc:42 \
    bootc install to-disk --wipe --bootloader systemd --filesystem xfs /dev/sdX

Suggested improvement

Something like:

error: systemd-boot is not supported with the ostree backend. Use --composefs-backend to enable systemd-boot support.

Or if the user explicitly asked for systemd-boot, auto-enable composefs (with a note).

Context

The current error is at install.rs:1818. The bail message is technically accurate but not actionable for users who don't know about the composefs/ostree backend distinction.