ginkgo/runner.py: print traceback when import/key error occurs

[hblanks]

This patch updates the ginkgo / ginkgoctl CLI's so that instead of:

[root@23.20.50.77][dev]# ginkgo ./deployment/collector.conf.py
Starting process with ./deployment/collector.conf.py...
usage: ginkgo [-v] [-h] [target]
ginkgo: error: Unable to load class path: blah.blah.foo:
No module named gar

You get:

[root@23.20.50.77][dev]# ginkgo ./deployment/collector.conf.py
Starting process with ./deployment/collector.conf.py...
usage: ginkgo [-v] [-h] [target]
ginkgo: error: Unable to load class path: blah.blah.foo:
No module named gar

Traceback (most recent call last):
  File "/home/hunter/ginkgo/ginkgo/runner.py", line 98, in load_class
    module = runpy.run_module(module_name + ".__init__")
  File "/usr/local/python-2.7/lib/python2.7/runpy.py", line 170, in run_module
    mod_name, loader, code, fname = _get_module_details(mod_name)
  File "/usr/local/python-2.7/lib/python2.7/runpy.py", line 101, in _get_module_details
    loader = get_loader(mod_name)
  File "/usr/local/python-2.7/lib/python2.7/pkgutil.py", line 456, in get_loader
    return find_loader(fullname)
  File "/usr/local/python-2.7/lib/python2.7/pkgutil.py", line 466, in find_loader
    for importer in iter_importers(fullname):
  File "/usr/local/python-2.7/lib/python2.7/pkgutil.py", line 422, in iter_importers
    __import__(pkg)
  ... <- your code here!
ImportError: No module named gar

In a sense, it'd be kind of nice if we didn't use argparse.error() for cases like this, when the issue is not an invalid argument on the command line, but rather a bug in the user's underlying ginkgo service.

[hblanks]

My commit has been updated so that it only prints tracebacks when the ImportError has occurred within the user's codebase. (i.e. a nested ImportError)

[progrium]

Good catch! Thanks Hunter