Warning if someone neither if the calendars are empty on first sync

[eingemaischt]

Some of our users want to sync a shared Caldav calendar with this realy cool tool, but they often don't understand that they have to create an empty calendar first.

So a warning if neither the remote nor local calendar are empty when the first sync starts (or while setup) would be nice. Otherwise they sometimes accidently upload their personal calendar...

(BTW: Is there any marking on the events in the vCal on the CalDav Server to recognize the events accidently uploaded by one person? An option to "remove calendar connection and delete all synced events may be neat to in this setting ;-) )