Allow the user to prevent the lookup from raising an exception

[tskirvin]

This is a quick attempt to merge in #12 into the current module world. It adds a 'raise_exceptions' option to lookup code to fail with an error message rather than failing out of the whole puppet process.

I haven't tested yet. Original comments, from @Magisus:

If a lookup function raises an exception, the whole catalog compilation will fail. This PR adds the ability to configure the lookup to not raise and instead simply log and return nil if it encounters an error, so that the users can structure their manifests to only conditionally depend on the result of the lookup.

We're not sure if this is useful or not, and are looking for use cases and feedback on the approach.

Fixes #13

[tskirvin]

The static validations stuff - I'm not sure if that's a problem with the test suite in general or in the code I merged in.

[bastelfreak]

CI fails because the ruby code isn't valid anymore, please have a look.