Update server.js

err应该是filter处理过的err，而不是起初的err
Add the cancelShutDownTimer hook

Add the cancelShutDownTimer hook,  lets coders can call it to cancel off
the default timer, and completely shutdown the server manually.
Merge pull request #642 from wuxian/patch-1

Update server.js
Merge pull request #644 from Cosmosbox/master

Add the cancelShutDownTimer hook