refactor: get rid of the return values from Command.run().

Output all the output to the logger from the command. Return anything on
success, raise an exception (caught from main.py) on error. Use the command's
object attributes to indicate whether we should continue with resolving or
setting an explicit command return value.
rename: YumTerm -> Term.
download_packages: propagate the librepo exception (RhBug:1035164)
drpm: add config options + docs, load updateinfo.xml
get_package_target(): don't use yum compatibility attribute
drpm: download delta packages if possible
tests: add repository with delta rpms
tests: MockBase() should load real repos, too.
tests: test rpm and drpm downloads from repo with deltas
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: fall back to rpm download on drpm errors
tests: test drpm to rpm failover
Hide the progress bar on failfast abort