catch malformed requests and return a 400

`decodeURI()` will throw when it has been given a malformed uri:

``` js
decodeURI('/%')
```

This patch fixes [issues/57](https://github.com/jesusabdullah/node-ecstatic/issues/57) by putting a try/catch guard around `decodeURI()` and returning a 400 error when `decodeURI()` throws.