FileStore::_collection_move_rename: propogate EEXIST

Previously, an EEXIST would get masked by the subsequent clone
operation.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend::rollback_stash: remove the correct shard

Fixes: #7616
Signed-off-by: Samuel Just <sam.just@inktank.com>