Fix mjsunit.js failure output

The bug was introduced in a previous refactoring:
https://chromium-review.googlesource.com/c/455555/14/test/mjsunit/mjsunit.js

R=mstarzinger@chromium.org
CC=gsathya@chromium.org

Change-Id: I49d2797c3ba834c89011910a81cffd9e119e719f
Reviewed-on: https://chromium-review.googlesource.com/593612Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47004}

test/mjsunit/mjsunit.js

@@ -286,6 +286,7 @@ var failWithMessage;
     } else {
       message += ":\nexpected:\n" + expectedText + "\nfound:\n" + foundText;
     }
+    return message;
   }
 
   function fail(expectedText, found, name_opt) {