Possible addition to tests for printing out custom errors

Decode_Result_Custom_test.re

/* Snip */
test("Enum failure can be mapped to string", () => {
    let message =
      switch (Color.decode(jsonInvalidStr)) {
      | Belt.Result.Ok(result) => result->Belt.Result.Ok
      | Belt.Result.Error(parseError) =>
        Decode_ParseError.toDebugString(
          x =>
            switch (x) {
            | `InvalidColor => (_json => "Invalid color")
            | `InvalidShape => (_json => "Invalid shape")
            | #DecodeBase.failure as f => DecodeBase.failureToString(f)
            },
          parseError,
        )
        ->Belt.Result.Error
      };
    expect(message) |> toEqual(Error("Invalid color"));
  });