Handling of exceptions from kiwix::Downloader::close()

[veloman-yunkan]

Fixes #694

If saving the downloader state during exit fails⁺ then, instead of crashing, the following error is reported on stderr:

ERROR: Failed to save the downloader state: Failed to serialize session to '/nonexistingdir/kiwix.session'.

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⁺It can happen, for example, if KIWIX_DATA_DIR is set to a non-existing directory which in addition cannot be created due to permissions.

[kelson42]

I have many remarks:

• We should habe a section about this ENv variable and how it works in the README
• I see no reason why we have such an ENV variable, why not kust removing it?

[veloman-yunkan]

@kelson42 Your remarks are valid but have nothing to do with this PR. kiwix::Downloader::close() can throw for a different reason (e.g. the directory ~/.local/share/kiwix being deleted while kiwix-desktop is running or the aria2 process dying).

[kelson42]

@veloman-yunkan OK, will open a dedicated ticket.