Returns a new CompletionStage that, when this stage completes
exceptionally, is executed with this stage's exception as the
argument to the supplied function. Otherwise, if this stage
completes normally, then the returned stage also completes
normally with the same value.
@param fn the function to use to compute the value of the
returned CompletionStage if this CompletionStage completed
exceptionally
@return the new CompletionStage
Returns a new CompletionStage that, when this stage completes exceptionally, is executed with this stage's exception as the argument to the supplied function. Otherwise, if this stage completes normally, then the returned stage also completes normally with the same value.
@param fn the function to use to compute the value of the returned CompletionStage if this CompletionStage completed exceptionally @return the new CompletionStage