better error description

app/scripts/locales/base.json

@@ -1,5 +1,18 @@
 {
-  "months": ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"],
+    "months": [
+        "January",
+        "February",
+        "March",
+        "April",
+        "May",
+        "June",
+        "July",
+        "August",
+        "September",
+        "October",
+        "November",
+        "December"
+    ],
     "monthsShort": ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"],
     "weekdays": ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"],
     "weekdaysShort": ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"],
@@ -192,6 +205,7 @@
     "openErrorFileNotFound": "File not found",
     "openListErrorBody": "There was an error loading file list",
     "openShowAllFiles": "Show all files",
+    "openFileNoCacheError": "File not found in the cache storage. This can happen because browser storage was cleaned up. To open the file, remove it from KeeWeb and add it again.",
 
     "detAttDownload": "Shift-click attachment button to download or",
     "detAttDelToRemove": "Delete to remove",

app/scripts/models/app-model.js

@@ -19,6 +19,7 @@ const AutoType = require('../auto-type');
 const Launcher = require('../comp/launcher');
 const RuntimeInfo = require('../comp/runtime-info');
 const PluginManager = require('../plugins/plugin-manager');
+const Locale = require('../util/locale');
 
 require('../mixins/protected-value-ex');
 
@@ -514,6 +515,9 @@ const AppModel = Backbone.Model.extend({
 
     openFileFromCache: function(params, callback, fileInfo) {
         Storage.cache.load(fileInfo.id, null, (err, data) => {
+            if (!data) {
+                err = Locale.openFileNoCacheError;
+            }
             new Logger('open', params.name).info('Loaded file from cache', err);
             if (err) {
                 callback(err);