storage: Don't fail during teardown when a fsys is already unmounted
storage: Don't fail during teardown when a fsys is already unmounted

This makes it possible to repeatedly apply a dialog, for example when
shrinking has failed and the user wants to try again with a different
size.

martinpitt/cockpit,garrett/cockpit,stefwalter/cockpit,garrett/cockpit,deryni/cockpit,andreasn/cockpit,deryni/cockpit,mareklibra/cockpit,martinpitt/cockpit,stefwalter/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,stefwalter/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,deryni/cockpit,andreasn/cockpit,mareklibra/cockpit,deryni/cockpit,cockpituous/cockpit,stefwalter/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit,cockpituous/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,deryni/cockpit,deryni/cockpit,andreasn/cockpit,martinpitt/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,cockpit-project/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,mvollmer/cockpit,mareklibra/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,andreasn/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mareklibra/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit