Show missing storage locations

[jsosulski]

Current behavior

When starting boostnote when a storage location is unavailable, the storage location does not show up at all. For instance, one of my storage locations is within a VeraCrypt container which is not always mounted.

In #2801 it was fixed that starting BoostNote without all storage locations available worked at all.

Expected behavior

After starting BoostNote, the name of the storage location should be shown, with an indicator showing that the actual folder on disk is not available.

Steps to reproduce

1. Create storage location that can be temporarily unavailable (usb stick, encrypted container).
2. Close BoostNote and unmount the media containing the storage location.
3. Start Boostnote. The storage location does not show up at all.

Environment

• Version : 0.11.17
• OS Version and name : Ubuntu 18.04