Remove Throwable interface from exceptions (port of #31 to 2.x)

src/ContainerExceptionInterface.php

@@ -5,6 +5,6 @@ namespace Psr\Container;
 /**
  * Base interface representing a generic exception in a container.
  */
-interface ContainerExceptionInterface extends \Throwable
+interface ContainerExceptionInterface
 {
 }

src/NotFoundExceptionInterface.php

@@ -5,6 +5,6 @@ namespace Psr\Container;
 /**
  * No entry was found in the container.
  */
-interface NotFoundExceptionInterface extends ContainerExceptionInterface
+interface NotFoundExceptionInterface
 {
 }