add translation for errors checking group name (#27196)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -6830,6 +6830,10 @@
     "id": "app.user.save.existing.app_error",
     "translation": "Must call update for existing user."
   },
+  {
+    "id": "app.user.save.groupname.app_error",
+    "translation": "Unable to determine if that username already exists"
+  },
   {
     "id": "app.user.save.username_exists.app_error",
     "translation": "An account with that username already exists."