Raise an error message if an encrypted EPUB with no embedded license is side-loaded

[llemeurfr]

This should not happen, but we've seen in the wild an encrypted EPUB (the output of the LCP encryption tool) being available for download on a library website. Thorium accepts to open it, but display garbage on screen.

Thorium should instead show an error message like "This encrypted EPUB does not contain a required LCP license; it is therefore invalid."