Remove a partial UFS/FFS snapshot if it fails to build successfully.

When taking a UFS/FFS snapshot, it may not succeed for example if the
filesystem is too full to hold it. When a snapshot is unable to be
successfully taken, the partial snapshot should be removed.

Reported-by: Peter Holm
Tested-by: Peter Holm
MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation