src/main/java/io/reactivex/Notification.java
@Nullable
    public Throwable getError() {
        Object o = value;
        if (NotificationLite.isError(o)) {
            return NotificationLite.getError(o);
        }
        return null;
    }
@Nullable
    public Throwable getError() {
        Object o = value;
        if (NotificationLite.isError(o)) {
            return NotificationLite.getError(o);
        }
        return null;
    }
Returns the container Throwable error if this notification is an onError
signal, null otherwise.
@return the Throwable error contained or null
@see #isOnError()