Add exponential retry logic if Github API returns 500

[the-gigi]

Yesterday we noticed that Github sometimes returns 500 (internal error) from API calls. For example:

Start processing validator: merge-gatekeeper....
Finish validator: merge-gatekeeper processing.
Error: validation failed, err: GET https://api.github.com/repos/invisible-platforms/invisible/commits/2aa531ef26cec2cbb26cd1f772b78f42b40ab064/check-runs?page=1&per_page=100: 500  []
failed to execute command: validation failed, err: GET https://api.github.com/repos/invisible-platforms/invisible/commits/2aa531ef26cec2cbb26cd1f772b78f42b40ab064/check-runs?page=1&per_page=100: 500  []

This PR solve the problem by retrying failed API calls.