bug #3543 Fix the definition of customizing form's global errors. (mtrojanowski)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix the definition of customizing form's global errors.

The documentation for customizing global errors was actually still referring to the way it was done in 2.0. (where two different blocks were used: `field_errors` and `form_errors`). Since 2.1. only `form_errors` is used and errors are differentiated using the `compound` variable (set to `true` for form errors).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3542

Commits
-------

6f6fcca Applied some changes according to comments.
e3b20d8 Minor rewording. Added tip about meaning of `compound` var and expanded the example code to contain `else` statements.
1e278c4 Added note about customizing errors for different field types.
236d06b Fix the definition of customizing form's global errors.

Author: weaverryan

cookbook/form/form_customization.rst

@@ -771,8 +771,17 @@ and customize the ``form_errors`` fragment.
     See :ref:`cookbook-form-theming-methods` for how to apply this customization.
 
 You can also customize the error output for just one specific field type.
-For example, certain errors that are more global to your form (i.e. not specific
-to just one field) are rendered separately, usually at the top of your form:
+To customize *only* the markup used for these errors, follow the same directions
+as above but put the contents in a relative ``_errors`` block (or file in case
+of PHP templates). For example: ``text_errors`` (or ``text_errors.html.php``).
+
+.. tip::
+
+    See :ref:`form-template-blocks` to find out which specific block or file you 
+    have to customize.
+
+Certain errors that are more global to your form (i.e. not specific to just one 
+field) are rendered separately, usually at the top of your form:
 
 .. configuration-block::
 
@@ -785,9 +794,46 @@ to just one field) are rendered separately, usually at the top of your form:
         <?php echo $view['form']->render($form); ?>
 
 To customize *only* the markup used for these errors, follow the same directions
-as above, but now call the block ``form_errors`` (Twig) / the file ``form_errors.html.php``
-(PHP). Now, when errors for the ``form`` type are rendered, your customized
-fragment will be used instead of the default ``form_errors``.
+as above, but now check if the ``compound`` variable is set to ``true``. If it
+is ``true``, it means that what's being currently rendered is a collection of 
+fields (e.g. a whole form), and not just an individual field.
+
+.. configuration-block::
+
+    .. code-block:: html+jinja
+
+        {# form_errors.html.twig #}
+        {% block form_errors %}
+            {% spaceless %}
+                {% if errors|length > 0 %}
+                    {% if compound %}
+                        <ul>
+                            {% for error in errors %}
+                                <li>{{ error.message }}</li>
+                            {% endfor %}
+                        </ul>
+                    {% else %}
+                        {# ... display the errors for a single field #}
+                    {% endif %}
+                {% endif %}
+            {% endspaceless %}
+        {% endblock form_errors %}
+
+    .. code-block:: html+php
+
+        <!-- form_errors.html.php -->
+        <?php if ($errors): ?>
+            <?php if ($compound): ?>
+                <ul>
+                    <?php foreach ($errors as $error): ?>
+                        <li><?php echo $error->getMessage() ?></li>
+                    <?php endforeach; ?>
+                </ul>
+            <?php else: ?>
+                <!-- ... render the errors for a single field -->
+            <?php endif; ?>
+        <?php endif ?>
+
 
 Customizing the "Form Row"
 ~~~~~~~~~~~~~~~~~~~~~~~~~~