@Nullable
    public static <T> T log(Throwable cause) {
        logger.warn("Unexpected exception from a completion action:", cause);
        return null;
    }
Logs the specified {@link Throwable}. For example:
<pre>{@code
CompletableFuture<?> f = ...;
f.exceptionally(CompletionActions::log);
}</pre>

@return {@code null}
core/src/main/java/com/linecorp/armeria/common/util/CompletionActions.java