Make build catch missing References

[sideshowbarker]

See whatwg/html#3074. We have lacked an entry for the Secure Contexts spec in the References section for a long while, during which we have had <ref spec="SECURE-CONTEXTS"> elements in the spec source referring to it. But the build has never emitted any error to tell us that the References section has the whole time lacked the necessary SECURE-CONTEXTS entry for those to resolve to.