Display human readable message instead of error codes

[nicolasgarnier]

When an error occurs currently the Error code is displayed. For instance when the account is disabled we display a popup with the following messsge:

Error code: auth/user-disabled  
Dismiss

Since this is supposed to be a reference implementation of the auth flow it would be nice to have all the known errors translated into a proper message for the user.

For instance forauth/user-disabled we should display something like "Your account has been disabled. Please contact the app's administrator."

[channing-google]

Fixed for 0.5.0.