Interactive mode should display help when a group is entered

Using interactive mode should feel fluid, but when exploring the command structure for a service, I don't find the terse nature of the argument exception usage being helpful.

## Currently
```
$ az interactive
az>> monitor
az monitor: error: the following arguments are required: subcommand
usage: az monitor [-h]
{alert,metrics,log-profiles,diagnostic-settings,autoscale-settings,activity-log}
...
```

## Desired
```
az>> monitor
interactive-mode showing help:
az monitor: Commands to manage Azure Monitor service.

Subgroups:
activity-log : Commands to manage activity log.
alert : Commands to manage metric-based alert rules.
autoscale-settings : Commands to manage autoscale settings.
diagnostic-settings: Commands to manage service diagnostic settings.
log-profiles : Commands to manage the log profiles assigned to Azure subscription.
metrics : Commands to view Azure resource metrics.
```