If a watched directory disappears, the watchers should be renewed.

[itkovian]

Watchers are registered with the kernel, but if the directory vanishes, the watcher stops getting notification for the FD it is subscribed for. The FD for the new directory is obviously different, meaning that we are now not watching that directory.

A fix could be to watch a removal event of the watched directory. For this, we probably need an extra watcher that will signal the threads to cease operations and restart them, while waiting for the new directories to become available.

If this happens frequently, it might induce a race condition, where we ask to watch locations that have since disappeared again. It should be OK to quit with an error in this case.