That's a good idea. It's always nice as a reader to get a head's up that an author's stuck or in conundrum. And to be honest, I think most readers will understand your situation.
And you are not really in a contract with your readers. Perhaps a social one, and yes, stopping a story could damage your reputation, but I really don't think so. In this age of the internet, comics and novels regularly die or go on hiatus as creator's priorities, schedule, and projects change. I think people will be sad, but it's not like this is unheard of, so I don't think most sane, rational human beings will hold it against you.
Also, some perspective: you are writing this for free and in your spare time. If you don't want to rewrite the story in order to post it, you absolutely don't have to. You are not legally obligated to finish anything, and social obligation to random readers shouldn't outweigh your own mental health and how you spend your time writing. If you really don't want to finish the unedited version of the story, I think if you explain your situation, most people will understand. As another poster suggested, they might like a summary of how things end.