Don't allow user to download book with empty chapters, or customize error message
Context
For an epub to be valid, it needs a table of contents. To have a table of contents, at least one chapter has to be named. Currently if you have one or many chapters in a book and they are all blank, you can click 'Download' for an epub export but will then get an error.
Proposal
Either:
- Disable epub 'Download' button when book has only empty chapters, as we currently do for totally empty books
- Allow the user to click 'Download' for epub when book has only empty chapters, but customize the error message to 'An epub cannot be downloaded because the book has no content.'
Design
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Implementation (if applicable)
[A description of the steps to implement the feature.]
Alternative approaches (if applicable)
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Open issues (if applicable)
[Links to and a discussion of related issues, if applicable.]