Added recovery in error 404 page

public/error_404.html

@@ -16,6 +16,7 @@
             <a href="https://stackedit.io"><img src="https://stackedit.io/res-min/img/logo-promo-128.png" width="316" height="64" /></a>
         </p>
         <p>404 &mdash; Page not found</p>
+        <p><a href="/recovery.html">Need StackEdit recovery?</a></p>
     </body>
 
 </html>

public/recovery.html

@@ -16,7 +16,15 @@
                 baseUrl: window.baseDir,
                 deps: ['main']
             };
-            setTimeout(function() {
+            var timer = 5;
+            function waitTimer() {
+                try {
+                    document.getElementById('timer').textContent = --timer;
+                }
+                catch(e) {}
+                if(timer) {
+                    return setTimeout(waitTimer, 1000);
+                }
                 var failure = false;
                 try {
                     // Try to use StackEdit dependencies to save a file
@@ -36,7 +44,8 @@
                     var elt = eltList[i];
                     show(elt);
                 }
-            }, 5000);
+            }
+            setTimeout(waitTimer, 1000);
 
             function show(elt) {
                 elt.className = elt.className.replace(/ hide/, '');
@@ -109,7 +118,7 @@
             </a>
             <h3>StackEdit recovery</h3>
             <p>
-                <strong>The download of your documents and settings will start in 5 seconds...</strong>
+                <strong>The download of your documents and settings will start in <span id="timer">5</span> seconds...</strong>
             </p>
             <p>To import your documents and settings back in StackEdit:</p>
             <blockquote>