Propagate login errors via backbone events.

Backbone (sensibly) doesn't fire a change event unless at least one
attribute has changed.  Therefore model.clear() might not fire if the
model was empty to start with.  Set up Account as { uninitialized: true}
and then call clear() on it if the login failed.
Stop listening to clicks on the login element if the account is logged in.

Setup the login element with class DV-UnknownAccount and only listen
for clicks on that class.  Once the login check completes
successfully, remove the class and change the wording.