Added missing localization strings for error page (#6026)

webapp/i18n/en.json

@@ -1297,6 +1297,10 @@
   "emoji_picker.search": "Search",
   "emoji_picker.symbols": "Symbols",
   "emoji_picker.travel": "Travel",
+  "error.local_storage.help1": "Enable cookies",
+  "error.local_storage.help2": "Turn off private browsing",
+  "error.local_storage.help3": "Use a supported browser (IE 11, Chrome 43+, Firefox 38+, Safari 9, Edge)",
+  "error.local_storage.message": "Mattermost was unable to load because a setting in your browser prevents the use of its local storage features. To allow Mattermost to load, try the following actions:",
   "error.not_found.link_message": "Back to Mattermost",
   "error.not_found.message": "The page you were trying to reach does not exist",
   "error.not_found.title": "Page not found",