add `warn` event for reporting page errors & verbose cli flag

page errors no longer fails the screenshot capturing.

@kevva can you review?
Thoughts on this output?

```
TypeError: 'undefined' is not a function (evaluating 'this.each(function(value, index) {
      results.push(iterator.call(context, value, index));
    })')
→ http://www.multimedia-browser.com/data/scripts/prototype.js?v=20: 867 (in function collect)
```