[internals] fix our base MessageBusError, which should inherit from E…

…xception instead of BaseException

src/pymessagebus/api.py

@@ -33,7 +33,7 @@ def has_handler_for(self, message_class: type) -> bool:
         pass
 
 
-class MessageBusError(BaseException, ABC):
+class MessageBusError(Exception, ABC):
     pass