Improve error reporting when svgo.json is broken

[bomberstudios]

See #21 for background info

[davemoz]

+1 for this.

I edited the json settings file once yesterday(only commented out one option), and today when I select "About SVGO Compressor" nothing happens :/ Would love to know more details about whether the file is broken or not.

[mathieudutour]

JSON doesn't support comments so you can't comment out a field. So i'd say the file is broken :)

[davemoz]

Ah, thanks! Changed it back and it's working. I guess this still applies to this issue though. More details when something is wrong, instead of simply not responding, would help diagnose the problem. Thanks again!