Poorly human-readable output when creation process fail because user already exists

[deusebio]

When we create a user in a namespace and with a service-account name that already exists, the CLI raises an exception which is however not too verbose

Steps to reproduce

1. Create a username with any name

python -m spark8t.cli.service_account_registry create --username my-spark-user

2. Re-create the user again

python -m spark8t.cli.service_account_registry create --username my-spark-user

Expected behavior

The second command fail but providing a proper humanly-readable exception

Actual behavior

The actual error is something like

Traceback (most recent call last):
  File "/home/deusebio/.pyenv/versions/3.10.6/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/deusebio/.pyenv/versions/3.10.6/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/cli/service_account_registry.py", line 201, in <module>
    raise e
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/cli/service_account_registry.py", line 195, in <module>
    main(args, logger)
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/cli/service_account_registry.py", line 115, in main
    registry.create(service_account)
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/services.py", line 1148, in create
    self.kube_interface.create(
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/services.py", line 881, in create
    self.exec(
  File "/home/deusebio/Canonical/data-platform/repos/spark/spark-k8s-toolkit-py/spark8t/services.py", line 726, in exec
    else subprocess.check_output(
  File "/home/deusebio/.pyenv/versions/3.10.6/lib/python3.10/subprocess.py", line 420, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/home/deusebio/.pyenv/versions/3.10.6/lib/python3.10/subprocess.py", line 524, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command 'kubectl --kubeconfig /home/deusebio/.kube/config  --namespace default  --context microk8s create serviceaccount my-spark-user  -o name ' returned non-zero exit status 1.
(

[deusebio]

This may be improved in #45. When an exception occurs, we now log stdout and stderr for the process