Check if instance exists before offering to edit a template for a new instance

Right now `limactl start` offers you to edit the template before starting the instance. But when it comes time to instantiate it, it finds that an instance with the same name already exists and throws away the edited template.

It would be more considerate to abort creation before letting the user edit the template:

```console
$ l start template://alpine
? Creating an instance "alpine" Open an editor to review or modify the current configuration
Waiting for Emacs...
FATA[0014] instance "alpine" already exists ("/Users/jan/.lima/alpine")
```