Running undefined subcommands gives a traceback

[Paul Larson]

Deepti pointed out to me today that if you try to run an undefined subcommand you get a traceback instead of a more useful error:

 bin/abrek results lists
Traceback (most recent call last):
  File "bin/abrek", line 30, in <module>
    exit_code = abrek.main.main()
  File "/home/ltcteam/abrek.sep15/abrek/main.py", line 31, in main
    main(argv)
  File "/home/ltcteam/abrek.sep15/abrek/command.py", line 69, in main
    return self.run()
  File "/home/ltcteam/abrek.sep15/abrek/command.py", line 76, in run
    self.name())
NotImplementedError: results: command defined but not implemented!