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The new 404 error page is used only when accessing ZIM file content
(i.e. as a response from the `/content` API endpoint).
One notable difference from the previous error page is that now no hint
is provided about whether the error is due to trying to access a
non-existent book/ZIM-file or non-existent resource inside a valid
book/ZIM-file (previously such a hint was present in the suggested
search URL). However, when displayed in the viewer this difference can
be seen in the viewer toolbar - book related buttons are hidden if the
URL points to a non-existent book.
This change breaks some unit tests. They will be fixed in a separate
commit.
