Hi,
there are two problems with the return/exit code of processing-java:

1/ the processing-java shell script does not exit with java's rc. The fix is easy:
java processing.mode.java.Commander "$@"
exit $? # add this line

2/ in Compiler.java/compile, the success boolean is set to true even if there are errors in the build. The success flag should be set to false again if it enters the error message processing.

A minor change like:
while ((line = reader.readLine()) != null) {
success = false; # add this line

could do the trick. Untested.

Thank you for this wonderful software