fix error message to make it more accurate

bindings/java/build-unix.xml

@@ -40,7 +40,7 @@
         <available file="./jni/.libs/libtsk_jni.dylib" property="tsk_dylib.present"/>
         <available file="./jni/.libs/libtsk_jni.so" property="tsk_so.present"/>
 		<available file="./jni/.libs/libtsk_jni.a" property="present"/>
-		<fail unless="present" message="Run make install on The Sleuthkit."/>
+		<fail unless="present" message="JNI native library not built."/>
    	<!-- Default location to find zlib and libewf. Overwritten by properties in makefile -->	
 	<property name="lib.z.path" value="/usr/lib"/>
 	<property name="lib.ewf.path" value="/usr/local/lib"/>