Clarify docs to avoid anti-pattern for .catch

    .then(null, handleError);

// By the way, this:

// However, this:

rp('http://google.com')

    .then(process)

    .catch(handleError);

// ... is equivalent to:

// ... is safer than:

rp('http://google.com')

    .then(process, handleError);

```

For more info on `.then(process).catch(handleError)` versus `.then(process, handleError)`, see Bluebird docs on [promise anti-patterns](http://bluebirdjs.com/docs/anti-patterns.html#the-.then).

### .finally(onFinished)

``` js