CoreCon: Improve error messaging

This adds another HRESULT_FACILITY to the conmanui whitelist, and adds
fallback to qt_error_string if the string is not found.

Change-Id: Ib4dddeca5cf5ba76e3c7fc5254f85e5a6949febe
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>