Error during flat-ui-icons build doesn't fail entire ant build?

[estanglerbm]

Got this error during ant build (the reason for the error is not important):

flat-ui-icons:
     [java] Error: Unable to access jarfile c:\somewhere\royale-asjs\frameworks\fontsrc\${env.ROYALE_HOME}\lib\mxmlc.jar
     [java] Java Result: 1

But the build still showed SUCCESS. Should the build error out on this kind of error?

Just wondering what is expected behavior.