Could it be to do with the fact that steal/buildjs has to be run from the parent folder, otherwise the above error is encountered? This is according to the latest JMVC documentation [link].
Edit: tested locally. The fix (to remove the above error) is to set the spawned child process' CWD to the parent (i.e. JMVC root), and set 'steal/buildjs' as the first argument.
However, the same (non-)result is achieved, in that nothing is produced.