storage: make exception inherits from StorageError
storage: make exception inherits from StorageError

This makes it easier to catch any storage error.

Change-Id: I4a092d95c9338b11cb5cf10e8ee9f8c81fce9226

gnocchixyz/gnocchi,sileht/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi