Fixes site_link_errors_total not counting errors

[AryanP123]

Fixes #2378

[AryanP123]

@c-kruse 's fix makes sense and tests pass.
As a possible follow-up we could allow the metric to always exist for every link by treating nil DownCount as 0 in Add/Update. Not required for this, but that way scrapers always see skupper_site_link_errors_total (at least at 0) instead of it only appearing after the router has reported a non-nil DownCount.

[fgiorgetti]

Patch worked ok for me.