[mod] UI: engine pref-page, about toolbox: add link to stats

In the preference page, in the 'about' toolbox of an engine, add a link to the
stats page of the engine, if the engine had one or more errors.

Condition is::

    reliabilities[<engine.name>].errors

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>