added "not connected to a google account" to registration problems list

This commit adds a third point to the list of possible problems in registration step 4 => if the registration with the push server (here: Google) fails this can have the only reason that you have no Google account attached to your device. 

I had this problem on all my devices (which were not connected to Google), such a hint would have saved me 10 hours of search & rescue. Perhaps other users have encountered the same problem.... my commit is a help to spot this problem.