sunbeam: greater detail on microstack bootstrap failure

[Heather Lanigan]

Running the sunbeam tutorial:

More detail on error would be helpful.

```
$ microstack bootstrap
Checking for presence of Juju ... done
Checking for presence of microk8s ... done
Checking for presence of openstack-hypervisor ... done
Error bootstrapping Juju
Traceback (most recent call last):
  File "/snap/microstack/316/lib/python3.8/site-packages/sunbeam/commands/juju.py", line 362, in run
    process = subprocess.run(cmd, capture_output=True, text=True, check=True)
  File "/usr/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/snap/microstack/316/juju/bin/juju', 'bootstrap', 'microk8s']' returned non-zero exit status 1.
Bootstrapping Juju into microk8s ... failed
Error: Command '['/snap/microstack/316/juju/bin/juju', 'bootstrap', 'microk8s']' returned non-zero exit status 1.
```
It's confusing when `/snap/microstack/316/juju/bin/juju` doesn't exist outside of running `microstack bootstrap`. Ran juju by itself and discovered that I'd skipped the step for storage, and was able to resolve the issue.
```
$ juju bootstrap microk8s
ERROR required storage addon is not enabled
```

[Billy Olsen (billy-olsen)]

Thanks for reporting this issue - we should definitely be looking to provide some more context that's returned rather than the current stack trace that's provided. Thanks for reporting this!

[Billy Olsen (billy-olsen)]

Triaging as high priority as I expect the extra bit of information presented will actually be able to help users help themselves in many cases, whereas the current experience - doesn't.