Can't upload an image using ckeditor after failing to add alt text

Problem/Motivation

When adding an image to an article or basic page via the body field, failing to enter alt-text triggers behaviour making it impossible to add an image or progress.

Steps to replicate.

1. Go to /node/add/article
2. Click the image icon in the editor
3. Upload an image
4. DON'T add alt text, click save.
5. View error saying alt text is required.
6. Add alt text, click save.
7. See image upload form, that doesn't work.

If you add alt text to begin with, it works fine.

Some screenshots to illustrate

Proposed resolution

Remaining tasks

User interface changes

API changes