errors importing commands are obfuscated

  affects testrepository

When running testrepository without python-fixtures installed I get a
strange error:

$ testr run
Traceback (most recent call last):
  File "/usr/bin/testr", line 26, in <module>
    sys.exit(run_argv(sys.argv, sys.stdin, sys.stdout, sys.stderr))
  File
"/usr/lib/python2.6/dist-packages/testrepository/commands/__init__.py",
line 178, in run_argv
    cmdclass = _find_command(cmd_name)
  File
"/usr/lib/python2.6/dist-packages/testrepository/commands/__init__.py",
line 49, in _find_command
    raise KeyError("Could not import command module %s" % modname)
KeyError: 'Could not import command module testrepository.commands.run'

Commenting out the exception handling I get:

Traceback (most recent call last):
  File "/usr/bin/testr", line 26, in <module>
    sys.exit(run_argv(sys.argv, sys.stdin, sys.stdout, sys.stderr))
  File
"/usr/lib/python2.6/dist-packages/testrepository/commands/__init__.py",
line 175, in run_argv
    cmdclass = _find_command(cmd_name)
  File
"/usr/lib/python2.6/dist-packages/testrepository/commands/__init__.py",
line 46, in _find_command
    _temp = __import__(modname, globals(), locals(), [classname])
  File
"/usr/lib/python2.6/dist-packages/testrepository/commands/run.py", line
27, in <module>
    from testrepository.testcommand import TestCommand, testrconf_help
  File "/usr/lib/python2.6/dist-packages/testrepository/testcommand.py",
line 18, in <module>
    from fixtures import Fixture
ImportError: No module named fixtures

It'd be nice if testrepository could give a saner error when fixtures
was missing.