Windows: more graceful handling external deletion of watched folders

In case a watched folder is deleted ReadDirectoryChangesW() fails with
an 'Access denied' (error code 5). In case of that error, it is better
to let the event emitter thread die. Otherwise it goes into a busy-loop,
polling the non-existant folder, eating 100% CPU.

Also in this patch, the error mentioned above is silenced in the
main loop to avoid the tracebacks which can not be handled by the
application anyway.