Cleanup bundle folder when creating a database fails

When creating a new database, if somehow a failure happens after a bundle folder is created, an empty bundle folder will be left behind. As a result, when trying to create a database again later, "WrongFormat" will always be thrown.

Basically if a failure happens after the following line when creating a database, the empty bundle folder will be left behind:

https://github.com/couchbase/couchbase-lite-core/blob/master/LiteCore/Database/DatabaseImpl.cc#L122