bugzilla: improve identifierNotForPull error
bugzilla: improve identifierNotForPull error

The `release-controller` is encountering an error for an invalid pull
identifier for a github link that is not a pull. This PR imporves
identification for non-pull github links to prevent that.

BenTheElder/test-infra,dims/test-infra,cjwagner/test-infra,michelle192837/test-infra,BenTheElder/test-infra,fejta/test-infra,BenTheElder/test-infra,jessfraz/test-infra,cjwagner/test-infra,kubernetes/test-infra,dims/test-infra,kubernetes/test-infra,michelle192837/test-infra,kubernetes/test-infra,fejta/test-infra,kubernetes/test-infra,michelle192837/test-infra,jessfraz/test-infra,fejta/test-infra,BenTheElder/test-infra,cblecker/test-infra,monopole/test-infra,dims/test-infra,jessfraz/test-infra,cjwagner/test-infra,monopole/test-infra,BenTheElder/test-infra,michelle192837/test-infra,jessfraz/test-infra,fejta/test-infra,michelle192837/test-infra,kubernetes/test-infra,dims/test-infra,fejta/test-infra,cblecker/test-infra,dims/test-infra,monopole/test-infra,dims/test-infra,BenTheElder/test-infra,cblecker/test-infra,jessfraz/test-infra,cjwagner/test-infra,fejta/test-infra,cblecker/test-infra,kubernetes/test-infra,cjwagner/test-infra,cjwagner/test-infra,cblecker/test-infra,monopole/test-infra,monopole/test-infra,monopole/test-infra,jessfraz/test-infra,cblecker/test-infra,michelle192837/test-infra