Signal Classed Condition Instead of just stop?

[brodieG]

@dselivanov asks:

@BrodieGaslam do you think it is feasible to make something similar to https://pydantic-docs.helpmanual.io on top of vetr?

Looks like a good idea to signal classed conditions, especially with increased support in R recently. All the pieces are there, we just have to extract them out.

This will also require #85 to be resolved.

I'm not actively adding features to vetr at the moment, so probably won't happen any time in the forseeable future, but if I were to get back to actively developing vetr this would be on the list of things to do.