Action does not report an error on push events if gh is not installed #18

I have been having some issues getting the action to work, not using the default Github actions image. It was giving me an error that I cannot upload the coverage report to a non-default branch with no PR, even though a PR for the branch exists.

After looking into it, I realized that I hadn't installed gh and this line just fails silently in that case. Could we add an error message or something if gh is not installed or throws an error?