Retry communication with GitHub if first attempt failed

[trz42]

Communicating with GitHub (e.g., adding a comment to a pull request) might fail for various reasons. For example, we once received a 401 response with the message Bad credentials. Then, retrying by restarting the bot AND redelivering the same event that eventually led to the comment action resulted in a successful operation.

Instead of just failing the bot could after waiting some time try to repeat the communication with GitHub. Only when it fails again, or repeatedly fails, it could stop trying and report the problem, e.g., via email.

[trz42]

#21 addresses only the Bad credentials issue. There may be other communication problems to which the bot should react more flexibly, i.e., by retrying, by reporting, ...

[trz42]

See also #30.