When listing cephfs storage-classes, handle exceptions

[addyess]

fix: Prevent charm errors when ceph-client relations are setup

Overview

Introduced in #35 , the cephfs storage classes class parameter_list raises ValueError when there is not enough information to build a list of storage classes. There are situations in normal operation of the charm (update-status or charm actions) when this isn't during a purging operation. In those situations, the best the manifest Manipulator can do is return an empty list.