Make echoerr, throw Error in liberator.open

common/content/liberator.js

@@ -1122,9 +1122,7 @@ const liberator = (function () //{{{
                         break;
 
                     default:
-                        // FIXME: should this really be a user-visible error? --djk
-                        liberator.echoerr("Exxx: Invalid 'where' directive in liberator.open(...)");
-                        return false;
+                        throw Error("Invalid 'where' directive in liberator.open(...)");
                 }
             }