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 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.

See Support question and answer: http://support.whispersystems.org/customer/portal/questions/6290806-verifying-number-hangs-in-step-4-registering-with-server-


(*) text on the screen is:
```

1. Connecting (ok)
2. Waiting for SMS verification (ok)
3. Generating keys (ok)
4. Registering with server... (fails: It hangs, spinning wheel turning)
```