Review CLI exit codes

I would like to see a toggle to verify if an instance exists
```
multipass info --quiet
```
returning 1 or 0 depending on existence or not.

Alternatively, this should work without failure:
```
multipass launch
multipass launch
```
Or add an `--exit-ok` type flag.

Start by tracing how the `multipass info --quiet ` and repeated `multipass launch ` commands determine their exit status. Review the comment thread for the unresolved choice between existence checking, idempotent launch, and an `--exit-ok` flag. Done means one documented behavior is implemented and its exit codes are verified for existing and missing instances.

Written by the indexing model from the issue text.