commands/run: require an argument
commands/run: require an argument

Currently, doing "cerbero run" without any command to run results in a
no-op with no output. However, running "no command" doesn't make much
sense, so cerbero should probably show an error message instead.

https://bugzilla.gnome.org/show_bug.cgi?id=763276

OptoFidelity/cerbero,centricular/cerbero,multipath-rtp/cerbero,nirbheek/cerbero,flexVDI/cerbero,GStreamer/cerbero,multipath-rtp/cerbero,centricular/cerbero,nirbheek/cerbero,centricular/cerbero,EricssonResearch/cerbero,justinjoy/cerbero,atsushieno/cerbero,shoreflyer/cerbero,OptoFidelity/cerbero,GStreamer/cerbero,fluendo/cerbero,atsushieno/cerbero,EricssonResearch/cerbero,justinjoy/cerbero,GStreamer/cerbero,justinjoy/cerbero,centricular/cerbero,fluendo/cerbero,GStreamer/cerbero,OptoFidelity/cerbero,centricular/cerbero,multipath-rtp/cerbero,shoreflyer/cerbero,nirbheek/cerbero,justinjoy/cerbero,atsushieno/cerbero,GStreamer/cerbero,fluendo/cerbero,shoreflyer/cerbero,fluendo/cerbero,OptoFidelity/cerbero,multipath-rtp/cerbero,flexVDI/cerbero,flexVDI/cerbero,fluendo/cerbero,flexVDI/cerbero,flexVDI/cerbero,EricssonResearch/cerbero,atsushieno/cerbero,multipath-rtp/cerbero,shoreflyer/cerbero,EricssonResearch/cerbero,shoreflyer/cerbero,atsushieno/cerbero,EricssonResearch/cerbero,nirbheek/cerbero