Gracefully handle invalid configuration files

Previously, `scss-lint` would raise "Report this bug" error when reading
an invalid configuration file. This isn't really a problem with
`scss-lint`, but the configuration file, so it should be more gracefully
handled.

Add support for reporting a nicer error message when invalid
configurations are provided.

Fixes #85

Change-Id: Ic4afe64a17fa24a04682003cd12d649efd54a261
Reviewed-on: http://gerrit.causes.com/36136
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>