MDL-32358 : numerical question: validation messages errors

The selected unit parameter is added answer paramter when renderer call
 $question->get_validation_error() so that it mimics the
is_complete_response(array $response) structure.

question/type/numerical/renderer.php

@@ -126,7 +126,7 @@ public function formulation_and_controls(question_attempt $qa,
 
         if ($qa->get_state() == question_state::$invalid) {
             $result .= html_writer::nonempty_tag('div',
-                    $question->get_validation_error(array('answer' => $currentanswer)),
+                    $question->get_validation_error(array('answer' => $currentanswer, 'unit' => $selectedunit)),
                     array('class' => 'validationerror'));
         }