Add error page for "combination of attributes is not unique"

[benjie]

{
  "data": {
    "onMachineUpdated": {
      "machine": null
    }
  },
  "errors": [
    {
      "message": "Attempted to call PgResource(latest_machines).get({serial_number}) at child field (TODO: which one?) but that combination of attributes is not unique (uniques: []). Did you mean to call .find() instead?",
      "locations": [
        {
          "line": 25,
          "column": 5
        }
      ],
      "path": [
        "onMachineUpdated",
        "machine"
      ]
    }
  ]
}

We have error pages like https://err.red/gasdd (which resolves to https://grafast.org/errors/asdd, produced by https://github.com/graphile/crystal/blob/main/grafast/website/src/pages/errors/asdd.mdx); would be good to have one with more details of this error.

[eeee88]

🌳