URL validator for custom buttons needs to validate HTTP response code

[christopher-rtf]

Issue:

If the URL is a valid URL and returns a 404 page, the validation is passing. It should fail with some sort of error (e.g. "page not found").

[sahil-rtf]

Fixed the issue by doing as follows:

1. Added 404 detection and 404s now fail with "Page not found (404)"
2. Added handling for other HTTP error statuses
3. Added a CORS proxy fallback to handle cases where direct fetch is blocked by CORS