os/FileStore: do not tolerate ENOENT on collection_add

If we get ENOENT on a collection_add that is a bad thing; do not ignore
the error.  This was hiding #4071 for some time.

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: tolerate ENOTEMPTY on RMCOLL on non-btrfs

We may have a sequence like:

 RMCOLL foo
 MKCOLL foo
 create object in foo
 <crash>

On replay, we need to tolerate the ENOTEMPTY unless/until we have a way
to do a replay guard-ish thing on the collection creation/deletion
operations.

Signed-off-by: Sage Weil <sage@inktank.com>