Detect permission problems

[cjyar]

According to https://developer.github.com/v3/issues/#create-an-issue:

Only users with push access can set labels for new issues. Labels are silently dropped otherwise.

Scenario:

• alertmanager-github-receiver is running as a user without permission to push to a repo.
• An alert is generated, and the program creates an issue. It tries to add labels to the issue, but no labels are added.
• Alertmanager periodically resends the alert. The next time alertmanager-github-receiver receives it, it checks to see if the issue already exists. It searches based on labels first, so it doesn't find the existing issue.
• alertmanager-github-receiver creates a duplicate issue.
• It does so again every time alertmanager resends the alert.

It seems like there are a few things we should/could do:

1. Warn users that they need to have push permissions for the service account.
2. Detect insufficient permissions and warn the user.
3. Refuse to create duplicate issues.

[stephen-soltesz]

re: 1) https://github.com/m-lab/alertmanager-github-receiver#create-github-access-token outlines what permissions are needed for the access token. Unfortunately, "push" is not mentioned on the token allocation page. A note in our readme that points to the link you put here would be welcome.

re: 2) yes, if the client doesn't have sufficient permissions it should not create any issues. That addresses 3) also.