pullrequests: detect invalid reviewers and raise HTTPBadRequest Normally, the creation of a pullrequest with invalid reviewers is not possible because the list of reviewers is populated from a form element that only shows valid reviewers. However, if creating a pullrequest through an API call, invalid reviewers can be specified but would not be detected. The reviewer would be encoded in the database as 'NULL'/None, and opening such a pull request would cause a server error. Instead, detect invalid reviewers at pullrequest creation/update time and raise HTTPBadRequest.