non-existent config file should throw error

[noblem]

The error thrown when specifying a non-existent config file are not consistent and clarifying. For example:

   %   bin/sample_report --config /no/such/file                                                                                            
   Required configuration variable is unset: dice.dir

    %  bin/gdc_dice --config /no/such/file
    Traceback (most recent call last):
     ...snip...
    TypeError: coercing to Unicode: need string or buffer, NoneType found

but should be something more simple, like:

Config file not found: /no/such/file