Drupal 8 features: Get a form element value VS set form error by element name

form_state_syntactic_ambiguities.php

<?php
$form['#tree'] = TRUE;

$form['part_of_the_form'] = [
  '#type' => 'container',
];

$form['part_of_the_form']['field_1'] = [
  '#type' => 'textfield',
  '#title' => 'Field-1',
];

$form['part_of_the_form']['field_2'] = [
  '#type' => 'textfield',
  '#title' => 'Field-2',
];

$form['part_of_the_form']['btn_submit'] = [
  '#type' => 'submit',
  '#value' => 'Submit this form',
];

//Get a value of form nested element (for example, text field):
$field1_value = $form_state->getValue(['part_of_the_form', 'field_1']);

//Show from error and highlight element with error we do this strange way:
$form_state->setErrorByName('part_of_the_form][field_1', 'Please, re-check Field 1.');