Error recovery

Usually, when a Telegram bot calls the configured endpoint at the webhook URL, getting an error on that call results in silence from the perspective of the end user. With other words, there’s no clear indication that user’s interaction resulted in an error.

Botpresto fixes this by sending an error back to the user, suggesting to try again later.

At the moment, only the connection error is being handled (e.g. when the endpoint isn’t responding, or there’s a network error).