better error when config.yml not provided

[mbacchi]

If 'etc/config.yml' is not present in the current working directory, and '-c PATH-TO-CONFIG' is not provided on the command line, the error is sortof unclear:

IOError: [Errno 2] No such file or directory: 'etc/config.yml'

You realize after a while that a config file is required but it takes a new user some guessing to get there. It should give you a clear error message that a config.yml file is required on the command line or in the CWD.

< viglesias> we can change the default to be:

1. use -c
2. if thats not there, check ./etc/config.yml
3. if thats not there check /etc/calyptos/config.yml

And I propose that the error should make clear the config file is required and not provided.

This can further be improved by adding information in the documentation and the --help ouptut that -c is required, along with -e (if indeed it is required.)