Issue 5549 - Implement missing error handlings for custom filter lists

locale/en_US/desktop-options.json

 {
   "options_page_document_title": {
     "description": "Options page title",
     "message": "Adblock Plus Options"
   },
   "options_page_title": {
     "description": "Page title in navigation sidebar",
     "message": "Settings"
   },
   "options_tab_general": {
@@ skipping 269 matching lines @@
   "options_customFilters_tip": {
     "description": "Custom filter widget(empty) hint text in Advanced tab",
     "message": "Filters you create will show up here."
   },
   "options_customFilters_start": {
     "description": "Custom filter widget(empty) button text in Advanced tab",
     "message": "Start writing my filter list"
   },
   "options_customFilters_learn": {
     "description": "Custom filter widget(empty) link text in Advanced tab",
-    "message": "Learn how to write filter lists"
+    "message": "Learn how to write filter lists."
   },
   "options_customFilters_edit_error": {
     "description": "Custom filters editing text in Advanced tab",
     "message": "Something's not right. Please check the following filters:"
   },
   "options_customFilters_edit_placeholder": {
     "description": "Custom filter widget(empty) placeholder text in Advanced tab",
     "message": "e.g. $filter$",
     "placeholders": {
       "filter": {
@@ skipping 150 matching lines @@
         "content": "$1",
         "example": "example.com"
       }
     }
   },
   "options_notification_hide": {
     "description": "Hidden text attached to the close button for screen readers of Notification",
     "message": "Close notification"
   }
 }