Output more meaningful error messages when JSON description do not conform to the expected schemas.

[elavoie]

The current JSON validator can be pretty cryptic when a description does not conform to an expected schema. Ex:

[
  {
    "field": "data",
    "message": "no (or more than one) schemas match",
    "value": {
      "type": "platform",
      "short-name": "Hanfeng’s MacBook Pro",
      "name": "Darwin-15.3.0-x86_64-i386-64bit",
      "cpu": "Intel Core i5",
      "gpu": "Intel Iris Graphics 6100",
      "memory": "8 GB",
      "os": "Mac OS X 10.11.3",
      "supported-languages": [
        "x86_64"
      ],
      "supported-formats": []
    }
  }
]

Rather than just bailing out, a second phase should check for common mistakes users make when writing description files, such as invalid short names, etc.