The documentation was unclear about what error codes would be returned
on attempting to open an empty or corrupt GVDB file. Previous versions
of the documentation incorrectly said that corrupt GVDB files were
considered equivalent to empty ones.

A recent commit has clarified the documentation to include its error
handling behaviour.

Update the two users of GVDB within GLib, GResource and GSettingsSource,
to follow this change, and add unit tests for them both.

Other users of the GVDB copylib will need to update their copy and make
appropriate changes if they have bugs in their handling of this
situation. dconf is one example of this. GVDB should be updated from
https://gitlab.gnome.org/GNOME/gvdb

.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#1454