Tarball step should report an error if the tarball is empty #344

In EESSI/software-layer#1913 an easystack file was not placed in the rebuilds directory, so the builds didn't do anything, and happily produced an empty tarball. The create_tarball.sh script (or one of the bot/* scripts) should just return an error in such cases to ensure that the build is reported as FAILED, which will prevent us from accidentally deploying empty tarballs.