Display context appropriate help and inform the user they can do '--help <command>'

Make it a bit clearer about how to get help, and if you get arguments incorrect.

Top level help
-------------------------
added the last line.
```
./bin/ansible-galaxy 
Usage: ansible-galaxy [--help] [init|info|install|list|remove] [options] ...

Options:
  -h, --help  show this help message and exit

See 'ansible-galaxy --help <command>' for more information on a specific
command.
```

Context help if you hit an error
-------------------------
```
./bin/ansible-galaxy init
Usage: ansible-galaxy init [options] role_name

Options:
  -h, --help            show this help message and exit
  -p INIT_PATH, --init-path=INIT_PATH
                        The path in which the skeleton role will be
                        created.The default is the current working directory.
  -s API_SERVER, --server=API_SERVER
                        The API server destination
  -f, --force           Force overwriting an existing role

See 'ansible-galaxy --help <command>' for more information on a specific
command.
No role name specified for init
```



Annoyingly parser = OptionParser(epilog='foo') doesn't run the the same filter as set_usage, so %prog isn't available