Found another case where onLogin could return early without
re-enabling the Login button.  Fixed in this commit.
In the case where the user specifies "[Use my default project]"
in the MASSIVE tab, and then fails to retrieve the user's
default project from Karaage, the Launcher was calling an
old method - die_from_main_frame which may be deprecated soon
and wasn't working as expected, so I modified this to do
a simple message dialog, as done for the case of an empty
username field.