Steps to reproduce

1. Go to any Talk page, e.g. your user talk page.
2. Write some text for a new topic, or a reply, but don't submit it yet.
3. Take the device offline.
4. Try submitting the message.

Expected results

1. A snackbar is shown with a meaningful error.
2. The user is able to try re-submitting the message.

Actual results

1. A snackbar is shown saying "Unknown error occurred."
2. The "Submit" button becomes disabled, and I'm no longer able to resubmit the message.