images: don't fail deleting when the storage delete fails
images: don't fail deleting when the storage delete fails

e.g. if someone deletes the zpool manually, they then can't delete an
image, because we can't detect the filesystem type for the path, because it
doesn't exist any more. Let's not fail so hard when we can't detect the
filesystem.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

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