Distinguish warnings and errors

[Robsteranium]

We could introduce distinguish critical problems from nice to haves by giving tests a :level: Swirrl/pmd-rdf-validations#20.

@RickMoynihan has suggested that we might handle those in the validator with command line options for:

--fail-level so you can at the command line turn :warning’s into failures and return a non-zero exit code if there are any
--output-level so you can control what level of warnings are run/output

[RickMoynihan]

Keywords for :level should be :warn and :error with an implict/default :level of :error.