Add a a helpful assert message for generated steps.

Reduce traceback verbosity for unimplemented tests, as well as explain why the test failed.
"Competes" with #254 .

Running tests shows 2 failures unrelated to this change.
```
Failing scenarios:
  features/formatter.json.feature:311  Use JSON formatter with feature and one scenario with steps
  features/scenario_outline.improved.feature:113  Select all generated scenarios of a Scenario Outline by File Location
```
both marked xfail.