Update UI to display user-friendly error message when claiming fails due to existing claim

Summary

When attempting to create an organization with a path that already exists in another cell, the Topology Service correctly returns a GRPC::AlreadyExists error. However, the UI currently does not display a user-friendly error message to inform users that the organization already exists.

Context

During verification of the transactional claiming behavior in #506 (comment 2878610051), we observed that:

  1. The backend correctly detects conflicts when trying to create an organization with a path that already exists
  2. The Topology Service returns GRPC::AlreadyExists exception
  3. The Rails logs show: "exception.class":"GRPC::AlreadyExists"
  4. However, the UI does not display a clear error message to the user

Current Behavior

When a user attempts to create an organization with a path that already exists:

  • The operation fails (as expected)
  • The UI shows a generic error or no clear feedback
  • Users are not informed that the organization path is already taken

Expected Behavior

When a user attempts to create an organization with a path that already exists:

  • The UI should display a clear, user-friendly error message such as:
    • "Organization path already exists"
    • "This organization name is already taken. Please choose a different name."
  • The error should be displayed in the organization creation form
  • The user should be able to easily retry with a different organization path

Acceptance Criteria

  • UI displays a user-friendly error message when GRPC::AlreadyExists is returned from the Topology Service
  • Error message clearly indicates that the resource path/name already exists
  • Error message is displayed in the appropriate location in the organization creation form
  • User can easily retry with a different organization path without losing other form data

Related Issues

Edited by Tarun Khandelwal