quilt push -a does not stop on errors

I found that a typo in xen-common-pq.git's series file cause "quilt push -a" as run by xenclient-pq.bbclass to print a warning (which is usually hidden by OE) and continue without error.

(OpenXT is currently using using quilt 0.51 from Jan 2012; qulit 0.63 was released on 7 May 2014. I inspected both versions and it seems there is no support for exiting with an error code in this situation.)
