`mesos completion` should give a nicer error message if called directly

[Sunil Shah]

Euch.

(env)➜  mesos-cli git:(nicer-help) ✗ mesos
Usage: mesos <command> [OPTIONS]

Available commands:
    cat
    completion
    config
    events
    execute
    find
    head
    help
    local
    log
    ls
    ps
    resolve
    scp
    ssh
    state
    tail
(env)➜  mesos-cli git:(nicer-help) ✗ mesos completion
Traceback (most recent call last):
  File "/Users/sunil/workspace/mesos-cli/env/bin/mesos-completion", line 9, in <module>
    load_entry_point('mesos.cli==0.1.4', 'console_scripts', 'mesos-completion')()
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 356, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 2431, in load_entry_point
    return ep.load()
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 2147, in load
    ['__name__'])
  File "/Users/sunil/workspace/mesos-cli/mesos/cli/cmds/completion.py", line 65, in <module>
    @cli.init()
TypeError: init() takes exactly 1 argument (0 given)
(env)➜  mesos-cli git:(nicer-help) ✗ mesos completion --help
Traceback (most recent call last):
  File "/Users/sunil/workspace/mesos-cli/env/bin/mesos-completion", line 9, in <module>
    load_entry_point('mesos.cli==0.1.4', 'console_scripts', 'mesos-completion')()
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 356, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 2431, in load_entry_point
    return ep.load()
  File "/Users/sunil/workspace/mesos-cli/env/lib/python2.7/site-packages/pkg_resources.py", line 2147, in load
    ['__name__'])
  File "/Users/sunil/workspace/mesos-cli/mesos/cli/cmds/completion.py", line 65, in <module>
    @cli.init()
TypeError: init() takes exactly 1 argument (0 given)