minor #13214 Adds exception template variable available in custom error pages (dbrumann)

This PR was squashed before being merged into the 4.4 branch (closes #13214).

Discussion
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Adds exception template variable available in custom error pages

Alongside `status_code` and `status_text` you also have access to `exception` template variable in custom error pages.

Commits
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60f3c8a Adds exception template variable available in custom error pages

Author: javiereguiluz

components/error_handler.rst

@@ -43,6 +43,8 @@ enable each of them separately.
     You should never enable the debug tools, except for the error handler, in a
     production environment as they might disclose sensitive information to the user.
 
+.. _turning-php-errors-into-exceptions:
+
 Turning PHP Errors into Exceptions
 ----------------------------------
 

controller/error_pages.rst

@@ -118,6 +118,17 @@ store the HTTP status code and message respectively.
     and its required ``getStatusCode()`` method. Otherwise, the ``status_code``
     will default to ``500``.
 
+Additionally you have access to the Exception with ``exception``, which for example
+allows you to output the stack trace using ``{{ exception.traceAsString }}`` or
+access any other method on the object. You should be careful with this though,
+as this is very likely to expose sensitive data.
+
+.. tip::
+
+    PHP errors are turned into exceptions as well by default, so you can also access these
+    error details using ``exception``. For more on this see
+    :ref:`Turning PHP Errors into Exceptions <turning-php-errors-into-exceptions>`
+
 Security & 404 Pages
 --------------------