Fix "Undefined index" PHP warnings on incorrect config
Fix "Undefined index" PHP warnings on incorrect config

I had set CALDAV_SUPPORTS_SYNC incorrectly (using SabreDAV 1.8.6
instead of the supported 1.9+) and CalDAVClient err'ed on me when it
tried to access its sync tokens, which it of course did not have.

This adds a clear warning message informing the user of a possible fix
and prevents the "Undefined index" warnings from occurring by checking
if a token is available.

fmbiete/Z-Push-contrib,mta59066/Z-Push-contrib,RavenB/Z-Push-contrib,Freeaqingme/Z-Push-contrib