fix unescaped exception in Multi.exec

index.js

@@ -1037,7 +1037,12 @@ Multi.prototype.exec = function (callback) {
                 if (typeof cur[cur.length - 1] === "function") {
                     cur[cur.length - 1](err);
                 } else {
-                    throw new Error(err);
+                  if (callback) {
+                      callback(new Error(err));
+                      return;
+                  } else {
+                      throw new Error(err);
+                  }
                 }
                 self.queue.splice(index, 1);
             }