message: fix(engine) Fix an error message.
  `error` is downcasted to `RuntimeError`, not to `Trap`. The error
  message is misleading.
diff: @@ -44,8 +44,9 @@ impl RuntimeError {
  // theory) we should only see user errors which were originally
  // created from our own `Trap` type (see the trampoline module
  // with functions).
  - // Self::new(format!("{}", error))
  - *error.downcast().expect("only `Trap` errors are supported")
  + *error
  + .downcast()
  + .expect("only `RuntimeError` errors are supported")
  }
  Trap::Runtime {
  pc,
  