Undefined callback error in mocha.run() on browser

Mocha.prototype.run(callback) provide an error status to the callback (0/1).
But this value is not forwarded to the browser callback.

    mocha.run(function(err) {
      console.log(err); // Always undefined
    })
Merge pull request #1216 from nopnop/patch-1

mocha.run() never return error status on browser