Add exception handling methods to CompletionStage and CompletableFuture

Summary

People have complained about lack of some exception-handling methods. This CR adds them.

Problem

The CompletionStage interface and CompletableFuture implementation lack some methods for dealing with exceptional outcomes that users need and expect based on presence of related non-exception-handling methods.

Solution

Add methods to cover all variants of exceptionally{Compose}{Async}:

exceptionally(Function<Throwable, ? extends T> f) // already exists
exceptionallyAsync(Function<Throwable, ? extends T> f);
exceptionallyAsync(Function<Throwable, ? extends T> f, Executor e);
exceptionallyCompose(Function<Throwable, ? extends CompletionStage<T>> f);
exceptionallyComposeAsync(Function<Throwable, ? extends CompletionStage<T>> f);
exceptionallyComposeAsync(Function<Throwable, ? extends CompletionStage<T>> f, Executor e);

Specification

/**
 * Returns a new CompletionStage that, when this stage completes
 * exceptionally, is executed with this stage's exception as the
 * argument to the supplied function, using this stage's default
 * asynchronous execution facility.  Otherwise, if this stage
 * completes normally, then the returned stage also completes
 * normally with the same value.
 *
 * @implSpec The default implementation invokes {@link #handle},
 * relaying to {@link #handleAsync} on exception, then {@link
 * #thenCompose} for result.
 *
 * @param fn the function to use to compute the value of the
 * returned CompletionStage if this CompletionStage completed
 * exceptionally
 * @return the new CompletionStage
 * @since 12
 */
public default CompletionStage<T> exceptionallyAsync
    (Function<Throwable, ? extends T> fn) {
    return handle((r, ex) -> (ex == null)
                  ? this
                  : this.<T>handleAsync((r1, ex1) -> fn.apply(ex1)))
        .thenCompose(Function.identity());
}

/**
 * Returns a new CompletionStage that, when this stage completes
 * exceptionally, is executed with this stage's exception as the
 * argument to the supplied function, using the supplied Executor.
 * Otherwise, if this stage completes normally, then the returned
 * stage also completes normally with the same value.
 *
 * @implSpec The default implementation invokes {@link #handle},
 * relaying to {@link #handleAsync} on exception, then {@link
 * #thenCompose} for result.
 *
 * @param fn the function to use to compute the value of the
 * returned CompletionStage if this CompletionStage completed
 * exceptionally
 * @param executor the executor to use for asynchronous execution
 * @return the new CompletionStage
 * @since 12
 */
public default CompletionStage<T> exceptionallyAsync
    (Function<Throwable, ? extends T> fn, Executor executor) {
    return handle((r, ex) -> (ex == null)
                  ? this
                  : this.<T>handleAsync((r1, ex1) -> fn.apply(ex1), executor))
        .thenCompose(Function.identity());
}

/**
 * Returns a new CompletionStage that, when this stage completes
 * exceptionally, is composed using the results of the supplied
 * function applied to this stage's exception.
 *
 * @implSpec The default implementation invokes {@link #handle},
 * invoking the given function on exception, then {@link
 * #thenCompose} for result.
 *
 * @param fn the function to use to compute the returned
 * CompletionStage if this CompletionStage completed exceptionally
 * @return the new CompletionStage
 * @since 12
 */
public default CompletionStage<T> exceptionallyCompose
    (Function<Throwable, ? extends CompletionStage<T>> fn) {
    return handle((r, ex) -> (ex == null)
                  ? this
                  : fn.apply(ex))
        .thenCompose(Function.identity());
}

/**
 * Returns a new CompletionStage that, when this stage completes
 * exceptionally, is composed using the results of the supplied
 * function applied to this stage's exception, using this stage's
 * default asynchronous execution facility.
 *
 * @implSpec The default implementation invokes {@link #handle},
 * relaying to {@link #handleAsync} on exception, then {@link
 * #thenCompose} for result.
 *
 * @param fn the function to use to compute the returned
 * CompletionStage if this CompletionStage completed exceptionally
 * @return the new CompletionStage
 * @since 12
 */
public default CompletionStage<T> exceptionallyComposeAsync
    (Function<Throwable, ? extends CompletionStage<T>> fn) {
    return handle((r, ex) -> (ex == null)
                  ? this
                  : this.handleAsync((r1, ex1) -> fn.apply(ex1))
                    .thenCompose(Function.identity())
    ).thenCompose(Function.identity());
}

/**
 * Returns a new CompletionStage that, when this stage completes
 * exceptionally, is composed using the results of the supplied
 * function applied to this stage's exception, using the
 * supplied Executor.
 *
 * @implSpec The default implementation invokes {@link #handle},
 * relaying to {@link #handleAsync} on exception, then {@link
 * #thenCompose} for result.
 *
 * @param fn the function to use to compute the returned
 * CompletionStage if this CompletionStage completed exceptionally
 * @param executor the executor to use for asynchronous execution
 * @return the new CompletionStage
 * @since 12
 */
public default CompletionStage<T> exceptionallyComposeAsync
    (Function<Throwable, ? extends CompletionStage<T>> fn,
     Executor executor) {
    return handle((r, ex) -> (ex == null)
                  ? this
                  : this.handleAsync((r1, ex1) -> fn.apply(ex1), executor)
                    .thenCompose(Function.identity())
    ).thenCompose(Function.identity());
}