There is currently no known z-library, and all known URLs are dead [1]. To avoid
interrupting automated updates, a connection error to a z-library is treated as
a *known error*, and the old properties of the z-library are retained.
[1] https://github.com/searxng/searxng/issues/3610
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>