miiocli: Provide an error message for unknown commands

[syssi]

This should be handled more nicely:

miiocli powerstrip --ip 192.168.130.89 --token 8f7537427792e82610162df211814ba8 foo
Traceback (most recent call last):
  File "/usr/local/bin/miiocli", line 11, in <module>
    sys.exit(create_cli())
  File "/usr/local/lib/python3.5/dist-packages/miio/cli.py", line 43, in create_cli
    return cli(auto_envvar_prefix="MIIO")
  File "/usr/local/lib/python3.5/dist-packages/miio/click_common.py", line 53, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1061, in invoke
    cmd_name, cmd, args = self.resolve_command(ctx, args)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1100, in resolve_command
    cmd = self.get_command(ctx, cmd_name)
  File "/usr/local/lib/python3.5/dist-packages/miio/click_common.py", line 221, in get_command
    cmd = self.commands[cmd_name]
KeyError: 'foo'