GitHub: Improve error message when PR creation fails
GitHub: Improve error message when PR creation fails

If this is incorrect, GitHub only gives an error message like
`{"resource":"PullRequest","field":"head","code":"invalid"}` which is
extremely hard to reason about without knowing some more details about
the original request.

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