Gracefully reject tasks which were spawned with a missing FS Factory

[000000003e966ea0] medialibrary medialibrary debug: src/MediaLibrary.cpp:303 initialize Initializing medialibrary...
[000000003e966ea0] medialibrary medialibrary debug: src/MediaLibrary.cpp:849 updateDatabaseModel Updating database model from 16 to 17
[000000003e966ea0] medialibrary medialibrary debug: src/MediaLibrary.cpp:364 initialize Successfuly initialized
[000000003e966ea0] medialibrary medialibrary debug: MediaLibrary successfully initialized
[000000003e966ea0] medialibrary medialibrary debug: src/MediaLibrary.cpp:370 start Starting medialibrary...
[000000003e966ea0] medialibrary medialibrary debug: src/factory/FileSystemFactory.cpp:109 refreshDevices Refreshing devices from IDeviceLister
[000000003e966ea0] medialibrary medialibrary debug: src/filesystem/unix/DeviceLister.cpp:120 listMountpoints Discovered mountpoint /dev/disk/by-partlabel/userdata mounted on /data (ext4)
[000000003e966ea0] medialibrary medialibrary debug: src/filesystem/unix/DeviceLister.cpp:120 listMountpoints Discovered mountpoint /dev/disk/by-partlabel/persist mounted on /persist (ext4)
[000000003e966ea0] medialibrary medialibrary debug: src/filesystem/unix/DeviceLister.cpp:120 listMountpoints Discovered mountpoint /dev/disk/by-partlabel/persist mounted on /var/chroot/qseecom/persist (ext4)
[000000003e966ea0] medialibrary medialibrary warning: src/filesystem/unix/DeviceLister.cpp:266 devices Failed to list devices: Failed to open /dev/disk/by-uuid: No such file or directory. Falling back to a dummy device containing '/'
[000000003e966ea0] medialibrary medialibrary debug: src/factory/FileSystemFactory.cpp:132 refreshDevices Caching device {dummy-device} mounted on file:///. Removable: false
[000000003e966ea0] medialibrary medialibrary debug: src/factory/FileSystemFactory.cpp:143 refreshDevices Done refreshing devices from IDeviceLister
[000000003e966ea0] medialibrary medialibrary debug: src/MediaLibrary.cpp:1685 refreshDevices Device {dummy-device} presence is unchanged
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:115 enqueue Queuing entrypoint  of type 1
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:75 discover Adding file:///mnt/home to the folder discovery list
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:115 enqueue Queuing entrypoint file:///mnt/home/ of type 0
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:75 discover Adding file:///mnt/home to the folder discovery list
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:115 enqueue Queuing entrypoint file:///mnt/home/ of type 0
[000000003e966ea0] medialibrary medialibrary debug: src/parser/ParserWorker.cpp:147 mainloop Entering ParserService [libvlccore extraction] thread
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/DiscovererWorker.cpp:144 run Entering DiscovererWorker thread
[000000003e966ea0] medialibrary medialibrary debug: src/discoverer/FsDiscoverer.cpp:143 reload Reloading all folders
No decoration plugins available. Running with no decorations.
[000000003e966ea0] medialibrary medialibrary debug: src/parser/ParserWorker.cpp:284 restoreTasks Resuming parsing on 1488 tasks
vlc: src/Folder.cpp:438: virtual const string& medialibrary::Folder::mrl() const: Assertion `!"Failed to find a FileSystemFactory for a known folder"' failed.
Aborted

This should just return TemporaryUnavailable instead of asserting.