Make it explicit that fetching or creating a session handler could throw an exception

This could happen when the handler is depending on a PHP extension, or connection to a database fails, or any other circumstance that may trigger an exception in the constructor of the session handler itself. We should be aware of this when using the session handler, therefore we update the phpdoc here to reflect that.

lib/SimpleSAML/SessionHandler.php

@@ -32,6 +32,8 @@ abstract class SessionHandler
      * to this function.
      *
      * @return \SimpleSAML\SessionHandler The current session handler.
+     *
+     * @throws \Exception If we cannot instantiate the session handler.
      */
     public static function getSessionHandler()
     {
@@ -126,6 +128,8 @@ abstract class SessionHandler
      * PHP session handler.
      *
      * @return void
+     *
+     * @throws \Exception If we cannot instantiate the session handler.
      */
     private static function createSessionHandler()
     {