How to tell where a Casper exit status code comes from?

How to tell where a Casper exit status code comes from?

I'm trying to use CasperJS to get snapshots of dynamic webpages (using
this Node server). Instead of returning the contents of a page, however,
Casper exits immediately with a status code of 127.
I would've initially guessed that this status code was specific to the
above server (and therefore not asked about it here), but 127 appears
nowhere in that sourcecode. Nor can I find it in the CasperJS source, the
Heroku buildpacks the server uses, or the PhantomJS source, though it's
possible it's in the latter and I missed it.
Put differently, is it the case that there is a default list of Casper
exit status codes? And if not, how might one track down the source of an
exit code?