Fail with a meaningful message.

Discussed on User-List: http://marc.theaimsgroup.com/?l=ant-user&m=116923425428838&w=2

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@498554 13f79535-47bb-0310-9956-ffa450edef68

build.xml

@@ -1439,13 +1439,15 @@
          Installs Apache Ant
        ===================================================================
   -->
-  <target name="install" if="ant.install">
+  <target name="install">
+    <fail message="You must set the property ant.install=/where/to/install" unless="ant.install"/>
     <antcall inheritAll="false" target="internal_dist">
       <param name="dist.dir" value="${ant.install}"/>
     </antcall>
   </target>
 
-  <target name="install-lite" if="ant.install">
+  <target name="install-lite">
+    <fail message="You must set the property ant.install=/where/to/install" unless="ant.install"/>
     <antcall inheritAll="false" target="dist-lite">
       <param name="dist.dir" value="${ant.install}"/>
     </antcall>