Aborting launch fails if workshop is running

It requires manual `lxc` commands but I think people are likely to run into this one:
```console
$ workshop launch --wait-on-error
error: cannot complete launch for "dev", execution is paused

To proceed, resolve the issue and run "workshop launch --continue dev"
To cancel and undo: "workshop launch --abort dev"
To view more information: "workshop tasks 39"
$ lxc start --project workshop.$(id -u) dev-$(< .workshop.lock)
$ workshop run debugging-script
$ workshop launch --abort
error: cannot perform the following tasks:
- Create new "dev" workshop (Instance is running)
"dev" launch aborted
```