Ignore err.status when it is less than 400.

Here you go @dougwilson!  Let me know if you want anything changed.

This addresses the following issues originally filed in [express](https://github.com/visionmedia/express):
https://github.com/visionmedia/express/pull/2194
https://github.com/visionmedia/express/issues/2193
https://github.com/visionmedia/express/pull/2192