gnome-documents: New error message for failing to load an e-book

[Debarshi Ray <rishi is lostca se>]

Hello everybody,

This cycle we grew a new application called gnome-books, which lives
in the gnome-documents module. Being a new application, it can only
load a few e-book formats at the moment. I would therefore like to add
a message in 3.15.91 to inform the user when this happens.

The new string and a screenshot are available on the bug:
https://bugzilla.gnome.org/show_bug.cgi?id=745172

Thanks,
Debarshi

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