HADOOP-2160. Fix build to fail when forrest fails.

git-svn-id: https://svn.apache.org/repos/asf/lucene/hadoop/trunk@601518 13f79535-47bb-0310-9956-ffa450edef68

build.xml

@@ -609,7 +609,7 @@
   <!-- ================================================================== -->
   
   <target name="docs">
-    <exec dir="src/docs" executable="forrest"/>
+    <exec dir="src/docs" executable="forrest" failonerror="true" />
     <copy todir="docs/">
       <fileset dir="src/docs/build/site/" />
     </copy>