Make the base exception extend throwable

This makes it more clear this interface can be 'caught'

Author: BackEndTea

src/Exception/ExceptionInterface.php

@@ -18,6 +18,6 @@
  *
  * @author Kévin Dunglas <[email protected]>
  */
-interface ExceptionInterface
+interface ExceptionInterface extends \Throwable
 {
 }