hg: lambda/lambda/jdk: Clarify message can be null as java.io package suggest otherwise.

[henry.jen at oracle.com]

Changeset: 3f0550ac4e9b
Author:    henryjen
Date:      2013-05-03 11:14 -0700
URL:       http://hg.openjdk.java.net/lambda/lambda/jdk/rev/3f0550ac4e9b

Clarify message can be null as java.io package suggest otherwise.

! src/share/classes/java/io/UncheckedIOException.java