Style issues in generated code

Noted by @noahdietz in comments on #166:

- https://github.com/googleapis/gnostic/pull/166#discussion_r415920689
Suggestion: put a return in this conditional block. `return x, compiler.NewErrorGroupOrNil(errors)`. This keeps the error flow indented but the following "green path" code path in the else at minimal indentation. Context: https://github.com/golang/go/wiki/CodeReviewComments#indent-error-flow

- https://github.com/googleapis/gnostic/pull/166#discussion_r415922830
Suggestion: put a continue in a !ok conditional rather than have the "green path" code be indented.