Prompt user before synchronization if storage path was changed and new path is empty

[jmehne]

vdirsyncer can rather unexpectedly (from a user perspective :) ) delete all events from a server:

1. Create a storage pair between an (empty) local and a remote storage.
2. Make an initial sync -> everything is fetched from the remote storage, great!
3. Change the local storage path in the config file and create that directory
4. Resync -> all events are deleted from the remote storage

Expected behaviour: Detect that the storage path was changed and fetch everything from the server again.

A very simple fix might be to detect that a storage directory is empty but a status file exists and to at least prompt the user if he is really sure to delete everything from the server.

[untitaker]

This is fixed now. vdirsyncer now requires --force-delete ... if it should synchronize two storages where one has been emptied.