Warn if installing to a small root partition

[davidmhewitt]

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