When bad options are passed from the build, intellij reports unhelpful output:

build.xml
script
Executing Ant target(s): build-all-classes
Building all classes...
/w/lt/9/testws/make/build.xml:210: exec returned: 2

Here, it's not clear to see what's happening. However, running ant manually reveals the problem:

build-all-classes:
     [exec] javac: invalid flag: --modulesourcepath
     [exec] Usage: javac <options> <source files>
     [exec] use --help for a list of possible options

BUILD FAILED
/w/lt/9/testws/make/build.xml:210: exec returned: 2

The IntelliJ custom ant logger should be enhanced to cover messages starting with 'javac:', as these typically indicates a failure.