storage/file: gracefully handle not mounted pool
storage/file: gracefully handle not mounted pool

Fixes QubesOS/qubes-issues#4668

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin