Issue 491 - Validate custom filters

_locales/en_US/messages.json

 {
   "add": {
     "description": "This is the label for the 'Add' buttons.",
     "message": "Add"
   },
   "add_domain_button": {
     "message": "Add domain"
   },
   "add_filter_button": {
     "message": "Add filter"
@@ skipping 170 matching lines @@
   "subscriptionSelection_subscriptionSelector_label": {
     "message": "Please choose a filter subscription from the list:"
   },
   "update_now": {
     "message": "Update now"
   },
   "whitelisted_domains": {
     "description": "This is a tab heading.",
     "message": "Whitelisted domains"
   },
+  "unexpected_filter_list_header": {
+    "message": "Filter list headers aren't allowed here"
+  },
   "invalid_css_selector": {
     "message": "$selector$ is not a valid CSS selector",
     "placeholders": {
       "selector": {
         "content": "$1"
       }
     }
   },
   "line": {
     "message": "Line $number$",
     "description": "Prepended to error messages to indicate the line number (e.g. \"Line 42: Some error on this line\")",
     "placeholders": {
       "number": {
         "content": "$1"
       }
     }
   }
 }