Show generic error when fetching site-url/wp-json namespaces is empty during site credentials login

[JorgeMucientes]

Related issue in Android: woocommerce/woocommerce-android#12813

When namespaces field is empty for site-url/wp-json the application password flow is triggered and then an error is shown:

emptyNameSpaces.mp4

We should probably show a generic error, like "Can't connect to your site ..." instead of triggering the application password.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.