[docs] Add cookbook recipe for the ErrorHandler

doc/cookbook/error_handler.rst

+How to convert errors to exceptions
+===================================
+
+Silex will catch exceptions that are thrown from within a request/response
+cycle. It will however *not* catch PHP errors and notices. You can catch them
+by converting them to exceptions, this recipe will tell you how.
+
+Why does Silex not do this?
+---------------------------
+
+Silex could do this automatically in theory, but there is a reason why it does
+not. Silex acts as a library, this means that it does not mess with any global
+state. Since error handlers are global in PHP, it is your responsibility as a
+user to register them.
+
+Registering the ErrorHandler
+----------------------------
+
+Fortunately, Silex ships with an ``ErrorHandler`` (it's part of the
+``HttpKernel`` package) that solves this issue. It converts all errors to
+exceptions, and exceptions can be caught by Silex.
+
+You register it by calling the static ``register`` method::
+
+    use Symfony\Component\HttpKernel\Debug\ErrorHandler;
+
+    ErrorHandler::register();
+
+It is recommended that you do this in your front controller, i.e.
+``web/index.php``.
+
+.. note::
+
+    The ``ErrorHandler`` has nothing to do with the ``ExceptionHandler``. The
+    ``ExceptionHandler`` is responsible for displaying caught exceptions
+    nicely.

doc/cookbook/index.rst

@@ -13,6 +13,7 @@ The cookbook section contains recipes for solving specific problems.
     form_no_csrf
     validator_yaml
     sub_requests
+    error_handler
 
 Recipes
 -------
@@ -32,3 +33,5 @@ Recipes
 * :doc:`How to use YAML to configure validation <validator_yaml>`.
 
 * :doc:`How to make sub-requests <sub_requests>`.
+
+* :doc:`How to convert errors to exceptions <error_handler>`.