code: public static void propagateIfPossible(@Nullable Throwable throwable) {
      propagateIfInstanceOf(throwable, Error.class);
      propagateIfInstanceOf(throwable, RuntimeException.class);
    }
docs: Propagates {@code throwable} exactly as-is, if and only if it is an
  instance of {@link RuntimeException} or {@link Error}.  Example usage:
  <pre>
  try {
  someMethodThatCouldThrowAnything();
  } catch (IKnowWhatToDoWithThisException e) {
  handle(e);
  } catch (Throwable t) {
  Throwables.propagateIfPossible(t);
  throw new RuntimeException("unexpected", t);
  }
  </pre>