Programmatically identify and increase unimplemented errors

[awoods187]

Use @mjibson's automated testing tool to randomly create syntax, test that it parses in Postgres, and then test it in CockroachDB. This will either result in implemented features, unimplemented features, or syntax errors. We should convert all syntax errors into unimplemented feature errors. More discussion in this doc:
https://docs.google.com/document/d/1pOQb8BVkWGuHtQFuZCqVf--nZiUO7027HqZymdGVbzI/edit#heading=h.gf6v9377rl7r