Add static isNotFound error checker to Router class

If used, it will allow the error message to change without inspecting
the error.message

lib/router.js

@@ -50,7 +50,10 @@ assign(Router.prototype, {
       return memo.concat(item)
     }
   }
-
 })
 
+Router.isNotFound = function isNotFound(err) {
+  return err instanceof Error && /not found/.exec(err.message) !== null
+}
+
 module.exports = Router

test/router.js

@@ -96,4 +96,16 @@ describe('Router', function () {
       done()
     })
   })
+
+  it('→ check not found error', function (done) {
+    var router = new Router()
+    router.route('/not/real/id', function (err) {
+      expect(Router.isNotFound(err)).toBe(true)
+      done()
+    })
+  })
+
+  it('→ check not found error on not real error', function () {
+    expect(Router.isNotFound(new Error('Not the missing error'))).toBe(false)
+  })
 })