Use Gherkin parser to give more meaningful error messages about file typos

[plocket]

The current cucumber messages about invalid feature files are confusing to authors. Since we're adding the Gherkin parser library for our constrained random answers generators Step, we could use it to pre-run it for all tests, catch the errors, and add give better descriptions before re-throwing the errors.

Do this before generating any files.

Make into new issue: Potentially, we could also use this to skip erroring files and still run the files that don't have errors. We can take a poll sometime on whether that would be useful to authors.