plugin loading failure just crash without any useful information

Hi, when a tempest plugin fails to load the backtrace doesn't help at all to known which plugins fail.

$ testr list-tests
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
OS_TEST_LOCK_PATH=${OS_TEST_LOCK_PATH:-${TMPDIR:-'/tmp'}} \
${PYTHON:-python} -m subunit.run discover -t ${OS_TOP_LEVEL:-./} ${OS_TEST_PATH:-./tempest/test_discover} --list
No handlers could be found for logger "tempest.test_discover.plugins"
--- import errors ---
Failed to call load_tests:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/unittest2/loader.py", line 136, in loadTestsFromModule
    return load_tests(self, tests, pattern)
  File "tempest/test_discover/test_discover.py", line 27, in load_tests
    ext_plugins = plugins.TempestTestPluginManager()
  File "tempest/lib/common/utils/misc.py", line 30, in getinstance
    instances[cls] = cls()
  File "tempest/test_discover/plugins.py", line 77, in __init__
    on_load_failure_callback=self.failure_hook)
  File "/usr/lib/python2.7/site-packages/stevedore/extension.py", line 93, in __init__
    verify_requirements)
  File "/usr/lib/python2.7/site-packages/stevedore/extension.py", line 171, in _load_plugins
    self._on_load_failure_callback(self, ep, err)
  File "tempest/test_discover/plugins.py", line 82, in failure_hook
    raise err
ImportError: No module named tests.tempest.plugin
Non-zero exit code (2) from test listing.

Cheers,