Comment 3 for bug 1717089

I could reproduce a similar issue (though not exactly the same error message):
ubuntu@epic-bear:~/spec$ mojo run
2018-03-26 06:29:57 [INFO] Using pre-existing container class lxd
/usr/lib/python2.7/dist-packages/pylxd/model.py:116: UserWarning: Attempted to set unknown attribute "description" on instance of "Container"
  key, self.__class__.__name__
2018-03-26 06:29:58 [INFO] Rsyncing /home/ubuntu/spec to /home/ubuntu/.local/share/mojo/lp1717089/xenial/lp1717089/spec
2018-03-26 06:29:58 [INFO] Retrieve the spec's manifest
2018-03-26 06:29:58 [INFO] Building resource tree
2018-03-26 06:29:59 [INFO] Downloading cs:ubuntu-12 from charm store to ubuntu
2018-03-26 06:29:59 [INFO] Downloading cs:nrpe-49 from charm store to nrpe
2018-03-26 06:29:59 [ERROR] Error calling external process
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 628, 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 346, in run_from_manifest
    manifest.run(project, workspace, args.stage)
  File "/usr/lib/python2.7/dist-packages/mojo/manifest.py", line 100, in run
    auto_repo=auto_repo)
  File "/usr/lib/python2.7/dist-packages/mojo/phase.py", line 255, in run
    collect_success = collect_config.build()
  File "/usr/lib/python2.7/dist-packages/codetree/config.py", line 271, in build
    results.append(f.result())
  File "/usr/lib/python2.7/dist-packages/concurrent/futures/_base.py", line 405, in result
    return self.__get_result()
  File "/usr/lib/python2.7/dist-packages/concurrent/futures/thread.py", line 55, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/dist-packages/codetree/config.py", line 115, in run_with_dependency
    return self.run()
  File "/usr/lib/python2.7/dist-packages/codetree/config.py", line 83, in run
    result = self.source.get(self.location, self.source_options)
  File "/usr/lib/python2.7/dist-packages/codetree/handlers/charm.py", line 85, in get
    if downloader.get(dest):
  File "/usr/lib/python2.7/dist-packages/codetree/handlers/charm.py", line 193, in get
    subprocess.check_output(cmd, stderr=subprocess.STDOUT, universal_newlines=True)
  File "/usr/lib/python2.7/subprocess.py", line 574, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
CalledProcessError: Command '('charm', 'pull', '--channel', 'stable', u'cs:nrpe-49', 'nrpe')' returned non-zero exit status 1
ubuntu@epic-bear:~/spec$