Fix call to the fail skylark global in junit.bzl
Fix call to the fail skylark global in junit.bzl

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

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