When get_juju_output fails permanently, stderr is not available

[Aaron Bentley]

For example:
http://reports.vapour.ws/releases/4269/job/functional-unregister/attempt/187

From another test that happens to run with --debug, we can infer that the error was:
flag provided but not defined: --models

This hinders diagnosing issues and writing good match rules.

The information is captured on CalledProcessError.stderr, so it should be easy to print when emitting the traceback.