Since we didn't have a refcat with coordinate errors, I couldn't write a test of jointcal's use of them. And thus I got it wrong: disabling config.astrometryReferenceErr resulted in an exception because the error field is coord_raErr not coord_ra_err!
Unrelatedly, but I might as well fix it as part of this work: it looks like jointcal is trying to apply colorterms when loading the astrometry reference catalog. That is both not necessary, and unhelpful when the astrometry refcat doesn't have useful photometry (e.g. Gaia). Might as well fix that on this ticket, too.