Do not fail when pending status cannot be added to PR

[mauritsvanrees]

See zopefoundation/Products.CMFUid#21 (comment).

During execution of a PR job, calling the PullRequest instance fails when adding a pending status check check to the PR. This is in case of a PR in the zopefoundation org, where Jenkins has no write permission.

It would be nice if this would warn and continue, at least for PR urls in the zopefoundation repo. Maybe always.

I suspect there will be a similar problem when reporting back the final result, but I don't know where that is.