Additional message for 500 or 503 response of HTTP bot

[sevenc-nanashi]

Related: #4041
It would be good if there are some external message on 500 or 503 like:

Interaction failed.
The application is unavailable.
From application:

("message" key of json, if there is)

Code	Message
500	There was an error in the application.
503	The application is unavailable. Please try again at (Calculated time from Retry-After header, if there is)

If there are some for 4xx code, it would be cool:

Code	Message
404, 405	Application developer didn't specified correct URL for bot.
403	You don't have permission to do this.
429	You are being rate limited.
Other	The application couldn't process interaction.