process: fix incorrect usage of assert.fail()

The message argument for `assert.fail()` is the third argument, not the
first. Correct minor misuse in internal module.

PR-URL: nodejs#6211
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/process/promises.js

@@ -13,7 +13,7 @@ function setupPromises(scheduleMicrotasks) {
     else if (event === promiseRejectEvent.handled)
       rejectionHandled(promise);
     else
-      require('assert').fail('unexpected PromiseRejectEvent');
+      require('assert').fail(null, null, 'unexpected PromiseRejectEvent');
   });
 
   function unhandledRejection(promise, reason) {