message: Improve error message on exception.
diff: @@ -210,7 +210,7 @@ impl std::fmt::Display for RuntimeError {
  } else if let Some(s) = data.downcast_ref::<&str>() {
  write!(f, "\"{}\"", s)
  } else if let Some(exc_code) = data.downcast_ref::<ExceptionCode>() {
  - write!(f, "\"{:?}\"", exc_code)
  + write!(f, "Caught exception of type \"{:?}\".", exc_code)
  } else {
  write!(f, "unknown error")
  }
  