code: public static RuntimeException propagate(Throwable throwable) {
      final Throwable t = Preconditions.checkNotNull(throwable, "throwable");
      if (t instanceof Error) {
        throw (Error) t;
      }
      if (t instanceof RuntimeException) {
        throw (RuntimeException) t;
      }
  
      throw new RuntimeException(t);
    }
docs: Propagates {@code throwable} as-is if it is an instance of
  {@link RuntimeException} or {@link Error}, or else as a last resort, wraps
  it in a {@code RuntimeException} then propagates.