if "charm pull" fails, it doesn't expose why

[Tom Haddon]

We recently saw the following error on a CI run with codetree:

2017-04-14 13:26:07 [ERROR] Error calling external process
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 620, in run_with_args
    args.func(args)
  File "/usr/lib/python2.7/dist-packages/mojo/utils.py", line 305, in wrapped
    return method(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 344, in run_from_manifest
    manifest.run(project, workspace, args.stage)
  File "/usr/lib/python2.7/dist-packages/mojo/manifest.py", line 99, in run
    auto_repo=auto_repo)
  File "/usr/lib/python2.7/dist-packages/mojo/phase.py", line 221, in run
    collect_success = collect_config.build()
  File "/usr/lib/python2.7/dist-packages/codetree/config.py", line 217, in build
    results.append(f.result())
  File "/usr/lib/python2.7/dist-packages/concurrent/futures/_base.py", line 397, in result
    return self.__get_result()
  File "/usr/lib/python2.7/dist-packages/concurrent/futures/_base.py", line 356, in __get_result
    raise self._exception
CalledProcessError: Command '('charm', 'pull', '--channel', 'stable', 'cs:~canonical-sysadmins/xenial/container-log-archive', 'container-log-archive')' returned non-zero exit status 1

Codetree should display the error here rather than just showing a traceback.

[Paul Collins (pjdc)]

It would be nice to get this into a release and deployed to is-mojo-ci. "charm show" seems to fail fairly often during CI runs and we don't know why.