More useful errors and logging when sync fails.

[JakePT]

Description of the Change

Adds more useful error messages with a troubleshooting URL to the log when a sync fails. Additionally the raw response is logged to the console as an error for easier debugging.

Closes #3131

How to test the Change

• When attempting a sync after logging out in another tab, the message "'Permission denied. Reload the sync page and try again." should appear.
• When attempting a sync, if a 500 error or similar occurs the message "Something went wrong" should appear, with a troubleshooting URL. Opening the sync page and then editing a plugin to deliberately cause a 500 error before clicking sync can help with testing this.
• If a 200 response is returned but with invalid JSON somehow, the message "Unable to parse response" should appear, with a troubleshooting URL. A quick way to test this is to modify action_wp_ajax_ep_index() to return a regular string.

For the last two messages an error should appear in the console with the raw response to the request.

Changelog Entry

Added - Extra debugging information in the browser console when syncing fails and more useful error messages with a troubleshooting URL.

Credits

@JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.