Show user-friendly error messages for Phone Auth

[samtstern]

In Phone Auth we are sending e.getLocalizedMessage() directly to an alert dialog:

FirebaseUI-Android/auth/src/main/java/com/firebase/ui/auth/ui/phone/PhoneVerificationActivity.java

Line 222 in 4e7bce1

showAlertDialog(ex.getLocalizedMessage(), null);

The problem with this is that error messages from the server frankly suck. So we need to do the following:

• Pick a set of known error codes and make human-friendly messages for them.
• Show 'unknown error' for the rest and only log the raw message to the console.

[samtstern]

The fix for this issue has been released in v2.1.0.