Fix sync_modes download and build with multiple packages

The functions download_tarball() and build_package() return early if
sync_mode is download and build respectively. Since calling clean_exit()
also breaks the process_targets() loop, return a non-zero value instead
and if one of them does not return zero the loop continues with the next
iteration.
Merge pull request #13 from bafain/master

Fix sync_modes download and build with multiple packages