Suppress `generator` error on panic

When a thread panics in a Shuttle test, [`generator` produces an error](https://github.com/Xudong-Huang/generator-rs/blob/master/src/gen_impl.rs#L558) that gets printed to stdout via `tracing`. This error is a red herring: it mentions `generator::gen_impl` and so suggests that maybe something went wrong in that code, when really it's just that a Shuttle thread panicked and that panic is being propagated. It would be nice to suppress this error so we don't lead users down the wrong debugging path.

Start by reading generator-rs's src/gen_impl.rs around line 558, then trace how Shuttle propagates a panic from a test thread and emits the tracing error. Done means the expected generator error is no longer printed for Shuttle thread panics, without hiding unrelated errors.

Written by the indexing model from the issue text.