The fail function takes an already formatted message,
it does not provide for formatting arguments, the
way Python's logger.log does, for example.
I did not find any other improperly made call to fail
in the rest of the bazlets project.
C.f. https://docs.bazel.build/versions/master/skylark/lib/globals.html#fail
Change-Id: If187b4d1f97ef331ead8a34f499c97cd8d5728f9