add catch block

Author: steff456

spec/_static/javascripts/version_dropdown.js

@@ -5,9 +5,8 @@ function assign_href( a, url, path ) {
         if( response.ok ){
             a.href = url + "/" + path;
         }
-        else {
-            a.href = url;
-        }
+    }).catch( error => {
+        a.href = url;
     });
 }