This consists of three commits:
koji_utils: fix return value in get_tagged_rpms()
Also rewrite one ugly line.
koji_utils: always create destination directory
Even when no RPMs are actually downloaded. This fixes issues with tools
or directives which then later expect the directory to exist.
To make things easier, stop wrapping filesystem errors into Taskotron
errors, but raise them as they are. That allows us to move a lot of
error checking and warning producing code into a single place.
koji_utils: add unit tests