Handle exceptions

Allow express to handle exceptions with default error handler. Now, we
can throw an exception into `handler` callback if limits reached.

@see https://expressjs.com/en/guide/error-handling.html#the-default-error-handler

lib/express-rate-limit.js

@@ -143,7 +143,8 @@ function RateLimit(options) {
           }
 
           next();
-        });
+        })
+        .catch(next);
     });
   }
 

test/express-rate-limit-test.js

@@ -658,4 +658,28 @@ describe("express-rate-limit node module", () => {
     rateLimit(opts);
     assert.deepEqual(opts, {});
   });
+
+  it("should handle exceptions", (done) => {
+    const store = new MockStore();
+    const app = createAppWith(
+      rateLimit({
+        max: 1,
+        store: store,
+        handler: () => {
+          const exception = new Error();
+          exception.code = 429;
+          exception.message = "Too many requests";
+          throw exception;
+        },
+      })
+    );
+    app.use((err, req, res, next) => {
+      res.status(err.code).send(err.message);
+      next;
+    });
+    goodRequest(done);
+    badRequest(done, () => {
+      done();
+    });
+  });
 });