Raised when an activity stub itself is wrong, rather than the activity failing.

A stub that raises is a legitimate way to exercise a workflow's failure branch, so Continuum.Runtime.Effect normalizes those into {:error, _} results the way the durable worker does. A stub with the wrong arity, or one returning a value that could never survive the journal, is a bug in the test instead — this exception is re-raised past that normalization so it surfaces as a test failure rather than as a plausible-looking activity error.