Add code to api.RaftSnapshot to detect incomplete snapshots

[ncabatoff]

An incomplete snapshot is one that doesn't include the final SHA256SUMS.sealed file.

[ncabatoff]

To test whether or not my implementation is a memory hog, I added the following to the end of OperatorRaftSnapshotSaveCommand.Run in operator_raft_snapshot_save.go:

	var m runtime.MemStats
	runtime.ReadMemStats(&m)
	c.UI.Info(fmt.Sprintf("totallocs=%d sys=%d", m.TotalAlloc, m.Sys))

I wrote 100k small keys to a raft cluster and took a snapshot with 1.8.1 and the above change:

totallocs=14887512 sys=76366856

With my branch and the above change:

totallocs=14945856 sys=75842568

(there's a bit of variability, but re-runs yielded similar numbers). The snapshot file was 11MB uncompressed, and I'm pretty sure if we held it all in memory those memory numbers would've gone up substantially. This confirms my understanding from code inspection of archive/tar, I just wanted to double check.

[ncabatoff]

Fixes #12168

[raskchanky]

Looks good, just a question about error handling

[sgmiller]

Looks good, love the injection of the error in the seal to trigger it.