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

tools/bzl/junit.bzl

@@ -43,11 +43,7 @@ def _AsClassName(fname):
         if findex != -1:
             break
     if findex == -1:
-        fail(
-            "%s does not contain any of %s",
-            fname,
-            _PREFIXES,
-        )
+        fail("%s does not contain any of %s" % (fname, _PREFIXES))
     return ".".join(toks[findex:]) + ".class"
 
 def _impl(ctx):