Ignore ConnectionError

To improve the signal-to-noise ratio and ensure that the pipeline only fails
when we made a mistake (and not when a website outside our control has an
outage), we ignore ConnectionError.

This is a minor change, because it fine-tunes the link checker of the
elastisys.io website.

linkchecker.conf

@@ -26,6 +26,7 @@ ignorewarningsforurls=
 ignoreerrors=
     # Avoid being someone else's uptime monitor :)
     ^https:// ^ReadTimeout
+    ^https:// ^ConnectionError
 
 [AnchorCheck]