Validate source options for cookbooks

Invalid source options in a Policyfile.rb will now be detected as the Policyfile.rb is read and reported as errors. To keep the tests passing I had to disable tests that specified chef-server as a cookbook source, since that isn't implemented in cookbook-omnifetch (and we probably won't implement that until Chef Server adds the /universe endpoint).
I tried this out, and I found that when you use `pending` in RSpec 3, the example isn't listed as "pending" in the test report. I prefer that these examples show up as pending in the test report so they're more visible, so I'm gonna stick with `skip` even though `pending` is probably what rspec wants me to do.