report error.stack to user

Hi, this is a neat idea!

Even though this is only a demo, it might still be a good idea to try reporting details to the user.

We know that JSON data can easily get invalid by a missed trailing comma, single-quote instead of double-quotes, etc.

While the reporting is not very detailed in such cases it might still be useful to the user to see something like this:

`
Error parsing json:SyntaxError: Unexpected token '    at Object.parse (native)    at HTMLButtonElement.doConvert (http://marianoguerra.github.io/json.human.js/js/demo.js:29:25)
`
Merge pull request #13 from anaran/master

report error.stack to user