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

page errors no longer fails the screenshot capturing.

-

Try it with:

```
pageres http://multimedia-browser.com 1000x1000 --verbose
```

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@kevva can you review?
Liking this output. Maybe remove the space between `:` and the line number (like Chrome dev tools), but that's no big deal. Or use wording like `→ file.js on line 867 (in function collect)`.