'CleanupOrFail': Cleanup but returns error if it would cleanup with -u

[chipaca]

I'd rather know early if there are snapshots that need cleaning up, rather than finding out while adding something unrelated. Keeps things tidy.

I thought of giving Cleanup() this behaviour directly, but suspected a breaking change like that would not be welcome. So instead I wrote its wacky sister function. If you see the name, cringe, and think of a better one, speak up -- I'm terrible at names.

HTH, HAND!