Ignore bad signature certificates with message

When list of signature certificates is processed any of them can result
with not using a cache while download, which is bad situation.

Just ignore bad certificates while checking the list, so any other good
ones can be used.

/reviewed-on !4
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:02

failed SlapOS.Eggs.UnitTest-Master.Python3

00:13:52