Add quit() function to ExternalBotHandler.

bot_handler.quit() should be used whenever a bot
wishes to terminate. This allows a flexible reaction
suited to the bot's environment: For external bots,
sys.exit() will be called, whereas for embedded bots,
different code can be executed.

zulip_bots/zulip_bots/lib.py

@@ -210,6 +210,10 @@ class ExternalBotHandler(object):
             raise PermissionError("Cannot open file \"{}\". Bots may only access "
                                   "files in their local directory.".format(abs_filepath))
 
+    def quit(self, message = ""):
+        # type: (str) -> None
+        sys.exit(message)
+
 def extract_query_without_mention(message, client):
     # type: (Dict[str, Any], ExternalBotHandler) -> str
     """

zulip_bots/zulip_bots/test_lib.py

@@ -45,6 +45,13 @@ class StubBotHandler:
         # type: (Dict[str, Any]) -> None
         self.message_server.update(message)
 
+    class BotQuitException(Exception):
+        pass
+
+    def quit(self, message = ""):
+        # type: (str) -> None
+        raise self.BotQuitException()
+
     def get_config_info(self, bot_name, optional=False):
         # type: (str, bool) -> Dict[str, Any]
         return None