Build doesn't fail when no deploy key is present

[asmeurer]

If there is no deploy key, it just says "Don't have permission to push. Not trying."

This is there so that builds don't fail on forks and other branches. Can we detect when it should be able to push, and fail if it cannot?

[asmeurer]

See for instance https://github.com/drdoctr/doctr/pull/241