Unrecognized command print traceback

[Robert Gildein]

I was trying to run `microstack list`, where `list` is not a valid command, and I got help along with a traceback error. I think I should see help or a list of commands with some error message, e.g. `list` is not a valid command.

What I get:
```bash
$ microstack list
usage: microstack <command> [<args>]

Available commands:
   init initialize a MicroStack node
   add-compute generate a connection string for a node to join the cluster
   launch launch a virtual machine

microstack

positional arguments:
  command A subcommand to run: {init, launch, add-compute}

optional arguments:
  -h, --help show this help message and exit
Traceback (most recent call last):
  File "/snap/microstack/233/bin/microstack", line 11, in <module>
    load_entry_point('microstack==0.0.1', 'console_scripts', 'microstack')()
  File "/snap/microstack/233/lib/python3.8/site-packages/microstack/main.py", line 37, in main
    raise Exception('Unrecognized command')
Exception: Unrecognized command
```

What would I expect:
```bash
$ microstack list
'list' is not a valid MicroK8s subcommand.
usage: microstack <command> [<args>]

Available commands:
   init initialize a MicroStack node
   add-compute generate a connection string for a node to join the cluster
   launch launch a virtual machine

microstack

positional arguments:
  command A subcommand to run: {init, launch, add-compute}

optional arguments:
  -h, --help show this help message and exit
```

microstack version:
```bash
snap list microstack
Name Version Rev Tracking Publisher Notes
microstack ussuri 233 latest/beta canonical✓ devmode
```