Errors from API returned as "Ok" when "Err" should be returned

[entropic77]

When the graphql-api returns an error, it will also return a list in data. This will cause elm-gql to return Ok, when it instead should return Err.

Here is an example query for Magento which will try create a user with erroneous data:

mutation createcustomer {
  createCustomerV2(
    input: {
      firstname: "a"
      lastname: "a"
      email: "a"
      password: "a"
    }
  ) {
    customer {
      email 
    }
  }
}

It will return:

{
	"errors": [
		{
			"message": "\"a\" is not a valid email address.",
                        # ...
		}
	],
	"data": {
		"createCustomerV2": null
	}
}

The problem is at

elm-gql/src/GraphQL/Engine.elm

Line 759 in 25171c7

(Json.Decode.field "data" (Json.Decode.null ()))

where oneOf fails to decode the first block, and instead returns the second block. This causes the function to return Ok instead of Err.