Detect cross-device uses of `--to`

When using `moss sync --to <dir>`, moss should detect early when `<dir>` is on a different device than the store, and error out with a helpful error message. Currently, it first does a bunch of work that's ultimately thrown away, before returning `Error: sync: client: blit: EXDEV: Cross-device link` (which is not a bad error message, it's just the wasted work that lead me to create this issue).