Bucket of honey is bubbling and smells sour — is it fermenting, and can it still be used for anything?

I think I have ruined a whole bucket of honey and I want to know if any of it can be saved. İzmir, second year, four hives. I extracted at the end of August, in a hurry, and took some frames that were only partly capped because I needed the supers off. Most of it went into jars and is fine. One 25 kg bucket got left on the shaded balcony with the lid just resting on top. Today I opened it: a layer of foam on top, small bubbles rising when I tilt it, and it smells sour — like cider, or bread dough. Borrowed the club refractometer and it reads 20%. I can't sell this, I understand. But is it wasted? People at the club said three different things: feed it back to the bees, make mead, use it for baking. One person said feeding fermented honey to bees will kill them. Which is right? And is there anything I should have done differently besides not taking uncapped frames? 😞