def add_error(expected_value, target_value)
      @errors[target_value.error_key] = <<-MSG

        Mismatch in field: '#{target_value.error_key}'
          expected: '#{expected_value.value}'
          received: '#{target_value.value}'

      MSG
    end
def add_error(expected_value, target_value)
      @errors[target_value.error_key] = <<-MSG

        Mismatch in field: '#{target_value.error_key}'
          expected: '#{expected_value.value}'
          received: '#{target_value.value}'

      MSG
    end
Adds an erorr to the list when a mismatch is detected
 @api private
 @return [String] the error message