Rework the whole unused get/setExitCause machinery

It is basically unused by now. Rework to be able to make use of it
later, no functional changes.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/ui/Curses.py

@@ -493,7 +493,7 @@ class Blinkenlights(UIBase, CursesUtil):
             # Request to quit.
             #TODO: this causes us to bail out in main loop when the thread exits
             #TODO: review and rework this mechanism.
-            currentThread().setExitCause('EXCEPTION')
+            currentThread().set_exit_exception(SystemExit("User requested shutdown"))
             self.terminate()
         try:
             index = acctkeys.index(chr(key))