FsDiscoverer: Refine behavior for removable devices

Don't delete a folder stored in a removable device if the root folder
can't be read. This is likely to be because the device was just mounted
and is not ready to be read from.
This is preventing any folder I indexed from my NAS to be removed when I
power it on:
MediaLibrary.cpp:2715 onDeviceMounted Device doop changed presence state: 0 -> 1
[...]
DiscovererWorker.cpp:527 runReloadDevice Reloading entrypoint on mounted device: smb://192.168.0.101:445/.../
FsDiscoverer.cpp:355 checkFolder Failed to browse smb://192.168.0.101:445/.../: Failed to browse network directory: Unknown error: Input/output error
FsDiscoverer.cpp:172 checkRemovedDevices The device containing smb://192.168.0.101:445/.../ is removable. Refreshing device cache...
[...]
MediaLibrary.cpp:870 deleteFolder deleting folder smb://192.168.0.101:445/.../