[index] Return error if no format is specified when requesting a bridge

index.php

@@ -134,7 +134,8 @@ try {
 			$bridge = substr($bridge, 0, $pos);
 		}
 
-		$format = filter_input(INPUT_GET, 'format');
+		$format = filter_input(INPUT_GET, 'format')
+			or returnClientError('You must specify a format!');
 
 		// DEPRECATED: 'nameFormat' scheme is replaced by 'name' in format parameter values
 		//             this is to keep compatibility until futher complete removal