message: stringify the error message
  i'm doing something stupid in my code and seeing [Object object]. This would help a lot.
diff: @@ -12,7 +12,7 @@ export default new GraphQLScalarType({
  if (!(v instanceof Date) && typeof v !== 'string' && typeof v !== 'number') {
  throw new TypeError(
  - `Value is not an instance of Date, Date string or number: ${v}`,
  + `Value is not an instance of Date, Date string or number: ${JSON.stringify(v)}`,
  );
  }
  @@ -26,7 +26,7 @@ export default new GraphQLScalarType({
  // eslint-disable-next-line no-restricted-globals
  if (Number.isNaN(v.getTime())) {
  - throw new TypeError(`Value is not a valid Date: ${v}`);
  + throw new TypeError(`Value is not a valid Date: ${JSON.stringify(v)}`);
  }
  return v.toJSON();
  