Currently, a non-!optional @extend will produce an error if its target exists but fails to unify with its selector. This doesn't really fit with the original goal of the "missing target" error for @extend, which was to help catch typos, and it complicates implementation because it means that unused extends can't be detected by simple set operations on extend targets and simple selectors. We should eliminate this error, and allow @extends to silently do nothing if they match but unification fails.