aptmirror does not display an error message when it fails

https://debusine.debian.net/debian/base/work-request/84904/ is an aptmirror work request and all that Debusine tells the user is that it is an "Error".

What would have been really useful here would be some indication that the cause for that failure is referencing a collection that has not yet been created (i.e. does not exist). The root cause for this error is a misconfiguration/user error. What is being reported here is the lack of diagnostic messages in the situation.