Missing config should cause more meaningful error message

[brejoc]

This is the strack trace the user sees, when path to the config is missing:

Traceback (most recent call last):
  File "/usr/local/bin/inv", line 9, in <module>
    load_entry_point('invoke==0.7.0', 'console_scripts', 'inv')()
  File "/usr/local/lib/python2.7/dist-packages/invoke/cli.py", line 263, in main
    dispatch(argv)
  File "/usr/local/lib/python2.7/dist-packages/invoke/cli.py", line 252, in dispatch
    dedupe=not args['no-dedupe']
  File "/usr/local/lib/python2.7/dist-packages/invoke/executor.py", line 76, in execute
    results[t] = t(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/invoke/tasks.py", line 70, in __call__
    result = self.body(*args, **kwargs)
  File "/src/_bnd/tasks.py", line 117, in prepare_paths
    config_values = get_config_vars(config)
  File "/src/_bnd/tasks_helper/config_generator.py", line 17, in get_config_vars
    stream = file(config, 'r')
IOError: [Errno 2] No such file or directory: 'configs/production.yaml'

Instead the user should see a more meaningful error message.