bot server: Raise BadRequest when bot code can't be imported.

This adds support to check and raise a BadRequest when either the
configuration of bot couldn't be found or the BotHandler code of bot
couldn't be fetched. There can be cases where flaskbotrc contains config
details of a bot, but the user hasn't added it's handler class code. This
fixes server to handle such cases, by reporting it to user.
This also fixes the response of bot server to make it possible to skip
the response message.

zulip_botserver/tests/test_server.py

@@ -35,10 +35,9 @@ class BotServerTests(BotServerTestCase):
                                         check_success=True)
         assert mock_ExternalBotHandler.called
 
-    def test_bot_not_supported(self):
+    def test_bot_module_not_exists(self):
         # type: () -> None
-        available_bots = ['testbot']
-        self.assert_bot_server_response(available_bots=available_bots,
+        self.assert_bot_server_response(bots_lib_module={},
                                         payload_url="/bots/not_supported_bot",
                                         check_success=False)