ErrorHandlerMiddleware::getRenderer($exception) needs interface

[dereuromark]

There is currently no contract for what the getRenderer must return.
If it, however, does not contain a render() method it will fail. Currently that is not checked at all - or enforced.
The IDE hints this already as yellow - for a good reason.

So

protected function getRenderer($exception) {}

should typehint as

/**
 * @return \Cake\Error\ErrorHandlerInterface
 */

And the handlers (custom or core) must implement this interface containing

• render()

method

Refs #9806

[markstory]

Another interface sounds reasonable to me.

[dereuromark]

The name is more appropriately named ExceptionRendererInterface. PR open.