feature #4168 [Components][Form] describe how to access form errors (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Components][Form] describe how to access form errors

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The ``getErrors()`` and ``getErrorsAsString()`` methods haven't been documented before. This pull request adds a short description for them. This forms as basis for the solution of #3660.

Commits
-------

5deccd2 describe how to access form errors

Author: weaverryan

components/form/introduction.rst

@@ -660,6 +660,33 @@ and the errors will display next to the fields on error.
     For a list of all of the built-in validation constraints, see
     :doc:`/reference/constraints`.
 
+Accessing Form Errors
+~~~~~~~~~~~~~~~~~~~~~
+
+You can use the :method:`Symfony\\Component\\Form\\FormInterface::getErrors`
+method to access the list of errors. Each element is a :class:`Symfony\\Component\\Form\\FormError`
+object::
+
+    $form = ...;
+
+    // ...
+
+    // an array of FormError objects, but only errors attached to this form level (e.g. "global errors)
+    $errors = $form->getErrors();
+
+    // an array of FormError objects, but only errors attached to the "firstName" field
+    $errors = $form['firstName']->getErrors();
+
+    // a string representation of all errors of the whole form tree
+    $errors = $form->getErrorsAsString();
+
+.. note::
+
+    If you enable the :ref:`error_bubbling <reference-form-option-error-bubbling>`
+    option on a field, calling ``getErrors()`` on the parent form will include
+    errors from that field. However, there is no way to determine which field
+    an error was originally attached to.
+
 .. _Packagist: https://packagist.org/packages/symfony/form
 .. _Twig:      http://twig.sensiolabs.org
 .. _`Twig Configuration`: http://twig.sensiolabs.org/doc/intro.html

reference/forms/types/form.rst

@@ -78,6 +78,8 @@ The actual default value of this option depends on other field options:
 .. include:: /reference/forms/types/options/empty_data.rst.inc
     :start-after: DEFAULT_PLACEHOLDER
 
+.. _reference-form-option-error-bubbling:
+
 .. include:: /reference/forms/types/options/error_bubbling.rst.inc
 
 .. include:: /reference/forms/types/options/error_mapping.rst.inc