stitch: Match empty errors in `checkError` unit tests
stitch: Match empty errors in `checkError` unit tests

It's convenient for an upcoming commit to check for the absence of
errors.

LuiseV/quilt,kklin/kelda,quilt/quilt,LuiseV/quilt,kklin/quilt,NetSys/quilt,kayousterhout/quilt,PenguinToast/quilt,ejj/quilt-dev,PenguinToast/quilt,kayousterhout/quilt,kelda/kelda,NetSys/quilt,LuiseV/quilt,tejasmanohar/quilt,kayousterhout/quilt,ejj/quilt-dev,kklin/quilt,ejj/quilt-dev,kelda/kelda,kklin/quilt,kklin/kelda,NetSys/quilt,tejasmanohar/quilt,NetSys/quilt,ejj/quilt-dev,quilt/quilt,kayousterhout/quilt,LuiseV/quilt,kklin/kelda,kklin/kelda,quilt/quilt,kelda/kelda,kelda/kelda,PenguinToast/quilt,tejasmanohar/quilt,quilt/quilt,kklin/quilt,PenguinToast/quilt,NetSys/quilt,tejasmanohar/quilt,PenguinToast/quilt,LuiseV/quilt