libebook: Don’t emit a warning on a dropped D-Bus connection in dispose

This occasionally and unavoidably happens in the libfolks unit tests,
where libfolks drops its final EBookClientView reference and proceeds to
kill the dbus-daemon. EBookClientView, however, keeps itself alive
internally in order to send a dispose() method call to the address book
server. This method call can return after the server has been killed,
and will return a G_IO_ERROR_CLOSED error in such cases. Ignore them.

https://bugzilla.gnome.org/show_bug.cgi?id=727784
libecal: Don’t emit a warning on a dropped D-Bus connection in dispose

See commit e31aae9c123da33a5613fa77cba55668e356f9ce.

https://bugzilla.gnome.org/show_bug.cgi?id=727784