In the changes to protobuf handling in
HBASE-15638, our TokenUtil class went from handling remote exception via a method that would throw IOException to one that simply returned it. In doing so, we ended up ignoring the results.
This causes the obtainToken method to start returning null where it would have previously thrown an IOException. It also causes obtainTokenForJob to still fail with an IOException, but instead of being the underlying remote problem now it's a generic "I didn't get a user token for some reason."
found via testing Pig with errors.