Add more detail to testdriver errors
Something like:
FAIL message: promise_test: Unhandled rejection with value: "error:
Action action_sequence failed"
…which doesn't give you much to go on.
Instead, let's change this to give detail of the expected exception:
FAIL message: promise_test: Unhandled rejection with value: "error:
Action action_sequence failed: move target out of bounds (500)"
If we have an unexpected exception, we instead output the full
traceback.