Critical Description
I've added a set of tests for https://github.com/apache/tinkerpop/pull/2036, with the CTR here https://github.com/apache/tinkerpop/commit/2c9e5300d6901c72e565f89e50ab0e4844ccd56e.
Ideally I want the Feature test to check that the exception fails with the intended error message, however, there seems to be a problem with the exception falling through when you have an empty graph and invalid maps to add just to g.V(['~label', vertex]). The intended errors appear to be null when you check with the step, but then gets printed after in the terminal output, which I wonder if it's not being caught in the toList() step, or if there is some race going on.