cerbero: Use FatalError in shell.download
cerbero: Use FatalError in shell.download

CommandError is only for command exceptions where we have a command
and a returncode.

Reported at https://gitlab.freedesktop.org/gstreamer/cerbero/commit/a6a5738c04c1f0437a9de766b654166f6445e80d#note_407161

GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero