If config.json is malformed sync fails with "unknown error"

[rufuspollock]

Once people start editing config.json they'll make mistakes like i did here -- note the missing comma on second line:

{
  "showEditLink": true
  "nav" {
    "title": "Datopian Tech"
  }
}

The result is the sync/build fails like this with an "unknown error". Would be better if some useful error message.

[rufuspollock]

Prioritizing as 2 🔥 until someone else reports.

[olayway]

FIXED