Add codes to GraphQL errors

[patch0]

This uses a GraphQL extension to provide a symbolic code when returning errors. This is documented in both GraphQL Ruby and in Apollo Server.

So for example, if you request a project you're not permitted to view, you'll now get:

{
  "data": {
    "project": null
  },
  "errors": [
    {
      "message": "An object of type Project was hidden due to permissions",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "project"
      ],
      "extensions": {
        "code": "FORBIDDEN"
      }
    }
  ]
}

Passing errors in this way allows deterministic handling of errors in the React app, as well as things like translations to be used in places where we'd maybe have relied on the message text sent back by the API.

What's changed

• Added EditorApiError class, which generates classes for various common error states, setting extensions => code with the appropriate code.
• Replaced most GraphQL::ExecutionErrors with these new error classes.
• For mutations, added an extra check to ready? to turn down requests for users
• Refactored the tests to be a bit more consistent on handling the no-op situation