Throw an exception instead of doing nothing if the auth source specified is...

Throw an exception instead of doing nothing if the auth source specified is invalid. This mimics the old behaviour.

lib/SimpleSAML/Auth/Default.php

@@ -21,9 +21,10 @@ class SimpleSAML_Auth_Default {
 		array $params = array()) {
 
 		$as = SimpleSAML_Auth_Source::getById($authId);
-		if ($as !== null) {
-		$as->initLogin($return, $errorURL, $params);
+		if ($as === null) {
+			throw new Exception('Invalid authentication source: ' . $authId);
 	}
+		$as->initLogin($return, $errorURL, $params);
 	}