Remove any .unwrap() and panic!s

[Jengamon]

For example: https://github.com/Jengamon/ruwren/blob/eb68c25c71bc2b86c7b13f0634855ddd329d8805/src/runtime.rs#L97

unwrapping or panicking in runtime.rs is UB as it crosses the C threshold. Remove these and replace with some kind of defaulting.