FTS: mossStore should delete half-written files on an error

Hi Sreekanth,

Related to https://issues.couchbase.com/browse/MB-27589

In that ticket, the cbft process ran out of disk space, so it rightly raised an error and exited, leading to a process restart. What needs to be double-checked is whether half-written files are getting properly deleted, such as during the restart/reopen logic here https://github.com/couchbase/moss/blob/master/store.go#L608. To help validate this is happening correctly, perhaps need to add more logging to help increase assurance on this, perhaps logging both all the files and on the files that are getting removed.