Prompt user to authenticate if an error occurs when adding the block

[faisal-alvi]

Description of the Change

This PR improves the error handling UX in the Jobber Gutenberg block by enhancing the message displayed when the iframe fails to load (due to a missing or invalid Jobber authentication or API response). Instead of showing only the error message, the block now provides contextual help and a direct link to the plugin's settings page (Jobber Settings) to assist the user in resolving common configuration issues like missing authentication.

Closes #11

How to test the Change

1. Ensure Jobber is not authenticated (e.g., clear saved credentials).
2. Add the Jobber block in the editor.
3. Select a form type ("Booking" or "Request").
4. You should see an error message with a link to "Jobber Settings".
5. Authenticate Jobber and retry to confirm the iframe renders properly.

Changelog Entry

Changed - Improved error messaging in Jobber block with contextual help and settings link when iframe fails to load.

Credits

Props @dkotter @faisal-alvi

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.