Undefined nested steps

The scenario in this commit highlights a regression about how we print snippets for 
undefined, nested steps.

But what's the right behaviour here? Previously, we treated these as the same as
undefined steps from features, but now I've looked into this, I'm not so sure.

If you have a nested step, surely you expect it to already match? If it fails
to match, I think you just want that to be a straight up failure, rather than
a regular "undefined" error that prints a snippet.

In the new 2.0 model, we know about undefined Gherkin steps as soon as we've 
compiled the mappings, but we can't know about nested steps until we execute
the action, so they're different. It feels like the model might be telling us
something.