ant の exec task で status code が non-zero のときにビルド失敗させるには failonerror="true"
<exec executable="python" failonerror="true"> とかすれば良い
https://stackoverflow.com/questions/7799778/ant-conditional-failure-upon-executable-failure
<exec executable="python" failonerror="true"> とかすれば良い
https://stackoverflow.com/questions/7799778/ant-conditional-failure-upon-executable-failure