docs: function ChecksumError(cause, actual, expected) {
      this.code = 'ChecksumError';
      if (expected === undefined) {
          expected = actual;
          actual = cause;
          cause = undefined;
      }
      assert.optionalObject(cause);
      assert.string(actual);
      assert.string(expected);
  
      var args = [];
      if (cause) args.push(cause);
      args = args.concat('content-md5 expected to be %s, but was %s',
          expected, actual);
      WError.apply(this, args);
  }