Improve log messages and add more tests

The messages should look less confusing now, as the phrasing for
expected errors has been changed.
4 jobs for master in 5 minutes and 53 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build Pkgsrc
passed build_pkgsrc:stretch #13701

00:00:29

 
  Build Pkg
passed build_pkg:stretch:amd64 #13702
amd64

00:00:46

passed build_pkg:stretch:arm64 #13703
arm64

00:05:00

 
  Upload Pkg
passed upload_pkg #13704

00:00:21