[MM-57877] Fix issue #26841 (#26956)

* Add new error message

* make i18n-extract

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Harrison Healey <harrisonmhealey@gmail.com>

server/i18n/en.json

@@ -9874,6 +9874,10 @@
     "id": "store.sql_team.save_member.exists.app_error",
     "translation": "A team member with that ID already exists."
   },
+  {
+    "id": "store.sql_team.save_team.existing.app_error",
+    "translation": "A team with this URL already exists."
+  },
   {
     "id": "store.sql_user.get_for_login.app_error",
     "translation": "Unable to find an existing account matching your credentials. This team may require an invite from the team owner to join."