hammer throws when asked to run unsupported system

See this example:

$ ./hammer.py ssh -s ubuntu -r 21.04 -p virtualbox
Traceback (most recent call last):
  File "/home/thomson/devel/kea/./hammer.py", line 2935, in <module>
    main()
  File "/home/thomson/devel/kea/./hammer.py", line 2918, in main
    ssh(args.provider, args.system, args.revision)
  File "/home/thomson/devel/kea/./hammer.py", line 2359, in ssh
    ve = VagrantEnv(provider, system, revision, [], 'kea', False)
  File "/home/thomson/devel/kea/./hammer.py", line 519, in __init__
    self.image_tpl = image_tpl = IMAGE_TEMPLATES[key][image_template_variant]
KeyError: 'ubuntu-21.04-virtualbox

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