Cleaner failure when missing config.yaml

[dfarrell07]

We currently fail with a FileNotFoundError when config.yaml is missing.

(vbot) [~bot]$ python start.py
Traceback (most recent call last):
  File "start.py", line 5, in <module>
    logger = lib.get_logger()
  File "/home/daniel/.virtualenvs/vbot/lib/python3.4/site-packages/libbot-0.1-py3.4.egg/libbot/lib.py", line 144, in get_logger
    config = get_config()
  File "/home/daniel/.virtualenvs/vbot/lib/python3.4/site-packages/libbot-0.1-py3.4.egg/libbot/lib.py", line 58, in get_config
    with open(config_file) as config_fd:
FileNotFoundError: [Errno 2] No such file or directory: 'config.yaml'

This should be made more graceful.