Bad username or password should be the indistinguishable

The error code returned by the webapp is @422@ in both cases, but depending on the context, we can tell if the user entered an incorrect password or an inexistent username. Look at the client file @srpauth.py@, exceptions @SRPAuthUnknownUser@ and @SRPAuthBadPassword@.

For more information look at: https://leap.se/code/issues/3656 and https://leap.se/code/issues/1810#note-2

(from redmine: created on 2013-09-16, closed on 2013-10-03, relates #3656 (closed), relates #1810, relates #4021)