add catch ajax error

src/ajax.js

@@ -5,6 +5,7 @@ export default function (url, options = {}) {
   xhr.send()
 
   return {
-    then: cb => xhr.addEventListener('load', cb)
+    then: cb => xhr.addEventListener('load', cb),
+    catch: cb => xhr.addEventListener('error', cb)
   }
 }

src/index.js

@@ -40,7 +40,8 @@ class Docsify {
   }
 
   load () {
-    ajax(`${this.loc}.md`).then(res => {
+    ajax(`${this.loc}.md`)
+    .then(res => {
       const target = res.target
       if (target.status >= 400) {
         this.render('not found')
@@ -52,6 +53,7 @@ class Docsify {
         }
       }
     })
+    .catch(_ => this.render('not found'))
   }
 
   render (content) {