Warn if installing to a small root partition

### Problem

The installer fails with a cryptic "unsquashfs" error if the root partition is too small to hold the uncompressed root partition.

There's a check in the installer that prevents you from selecting a disk smaller than 15GB. But if you select small partitions on an otherwise larger disk, the install will fail with this error and it's hard to figure out why.

### Proposal

Before installation, check the size of the root partition we're going to install to, and warn/fail if it's less than what we require.

### Prior Art (Optional)

_No response_