Give better message in case of missing param.

We've had a number of user trip up because you have to specify
e.g. --foo-bar rather than --foo_bar. So I think this'll help
avoid the common confusion.

Example error message looks like:

luigi.parameter.MissingParameterException: No value for
'date_interval' (--date-interval) submitted and no default value
has been assigned.
Merge pull request #27 from jcrobak/better-error-msg

Give better message in case of missing param.