Include previous execption for repair steps that don't exist

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

lib/private/Repair.php

@@ -149,7 +149,7 @@ class Repair implements IOutput {
 						throw new \Exception("Repair step '$repairStep' can't be instantiated: " . $e->getMessage(), 0, $e);
 					}
 				} else {
-					throw new \Exception("Repair step '$repairStep' is unknown");
+					throw new \Exception("Repair step '$repairStep' is unknown", 0, $e);
 				}
 			}