Recently merged https://gerrit.wikimedia.org/r/#/c/321406/ probably breaks like a dozen things in UploadWizard (the announcement https://lists.wikimedia.org/pipermail/mediawiki-api/2016-December/003835.html mentions action=upload in particular). We should go through all of our error handling code and verify what still works right and what needs fixing. Also, we need to do this before next week's deployment. :/
Not at this point, and it doesn't seem like much (if anything) will be broken - BC is quite good. But I'm sure I can use some help in testing of all kinds of possible errors next week :p
So, just to be clear, we don't actually have any compatibility issues at the moment, and we don't need to do anything before the deployment? This probably doesn't have to be "High" priority then.
It looks like I actually misread the warning in the announcement:
action=upload now reports optional stash failures as an array under key 'stasherrors' rather than a 'stashfailed' text string.
This talks about optional stash failures, which is something we don't use (it's about the case where you try to upload and publish the file with a single action=upload, and you get an error which prevents publishing but allows the file to be stashed; if the stashing also fails, that's called an optional stash failure). We do uploading and publishing in separate action=upload queries.