Initializes the cause of this throwable to the specified value. (The
cause is the throwable that caused this throwable to get thrown.)
This method can be called at most once. It is generally called from
within the constructor, or immediately after creating the throwable. If
this throwable was created with JAXRException(Throwable) or
JAXRException(String,Throwable), this method cannot be called even once.
Parameters: cause - this