UX Enhancement: Improve Error Message for Duplicate Helpful Links

[jtucholski]

Description:
Admins encounter an error when trying to create a new Helpful Link with the same URL as an existing one.

Once PR #635 is merged, users will see an error toast message that states:
Error adding helpful link.

This occurs because helpful_links.go checks for an existing Helpful Link with the same URL and returns a "Link already exists" error to the client.

Proposed Solution:
Since there may be many Helpful Links, we should provide a clearer error message to users. Instead of the generic message, display "Link already exists". This will help users understand what went wrong and avoid repeated attempts to add the same link.