Add support for 422 responses

[whiteinge]

Would be helpful to display the error message from GitHub at the CLI. E.g.

{
  "message": "Validation Failed",
  "errors": [
    {
      "resource": "Milestone",
      "code": "missing_field",
      "field": "title"
    },
    {
      "resource": "Milestone",
      "code": "missing_field",
      "field": "slug"
    }
  ],
  "documentation_url": "https://developer.github.com/v3/issues/milestones/#create-a-milestone"
}