Enhance RSOE error screen with emergency download tiddlers button

[Jermolene]

This PR adds a button to the "red screen of embarrassment" error dialogue to allow the contents of the tiddler store to be downloaded as a JSON file.

It currently looks like this, but I am intending to improve the styling:

Trying it out

The preview has been rigged so that clicking on any tiddler link will initiate an error.

1. Visit the preview
2. Click a tiddler link
3. See the RSOE and click the emergency download button

Progress

• [x] Basic functionality
• [ ] Styling