cerbero: Exit after selecting 'shell' on build failure
cerbero: Exit after selecting 'shell' on build failure

Previously Cerbero would exit start_cooking() and continue to try and
do packaging, which would fail in a confusing way.

nirbheek/cerbero,fluendo/cerbero,atsushieno/cerbero,GStreamer/cerbero,atsushieno/cerbero,GStreamer/cerbero,GStreamer/cerbero,fluendo/cerbero,atsushieno/cerbero,nirbheek/cerbero,fluendo/cerbero,atsushieno/cerbero,nirbheek/cerbero,fluendo/cerbero,GStreamer/cerbero,nirbheek/cerbero,atsushieno/cerbero,GStreamer/cerbero,fluendo/cerbero