Report operational errors

[dontcallmedom]

There are at least two ways the checker can fail to process a pull request:

• it doesn't recognize the repo (e.g. after a name change ash-nazg should handle repository name changes #64)
• the secret used to verify the origin of the check is wrong (which has happened a number of times, although I'm not sure how)

… and probably more. Right now, these failures are simply logged as a webook failure - useful for debugging, not for noticing there is a configuration issue.

These should instead be reported - either on the pull request itself, or on this repo (probably both - e.g. it could stay in "pending" state on the pull request status with an appropriate message, while reporting the issue here).

[deniak]

I'll try to work on that in the next few days

[dontcallmedom]

It would be useful to also report issues on webhooks with wrong secrets, a frequently occurring issue (although I've never quite figured how that happens)