Returns the exception thrown by the base computation, or a {@code CancellationException} if cancelled, or {@code null} if none or if the method has not yet completed.
@return the exception, or {@code null} if none
See Implementation
Returns the exception thrown by the base computation, or a {@code CancellationException} if cancelled, or {@code null} if none or if the method has not yet completed.
@return the exception, or {@code null} if none