Handling validation errors

This is an excellent question. I don't have time right now to dive into details. But I can tell you 3 things:

1. If you feel this is not quite an ideal format, then you're absolutely right!
2. Unfortunately, http://jsonapi.org/format/#errors-processing does not at all specify how this should work — it leaves it up to the implementation.
3. This is also a problem in Drupal core's rest module, and in fact, the jsonapi module pretty much does the same thing. #1916302: RFC 7807: "Problem Details for HTTP APIs" — serve REST error responses as application/problem+json exists to improve that.

Looking forward to what the other JSON API maintainers have to say about this one :)

Thanks for taking the time to describe the problem so clearly by the way! That alone is a valuable contribution! 👍❤️

I realize I forgot to answer your concrete question:

For now, the only idea I have is to split that key to isolate "title" and map it back to the field in the UI.

Correct, you would indeed have to parse that string. That's today's reality. Hopefully we can improve that in the future!

I kind of feel better, I thought I was missing something very obvious :)

Right, I totally understand that — that's how I felt too the first time I saw it: I must be doing something wrong, because this seems … very very far from the ideal.

That's still true to this day. Remember, Drupal is FLOSS, so some things just don't get done unless somebody who cares about them finds the time to get it done. The good thing is that this is an area where you can have a lot of impact: you can help shape how this will/should work. With code/patches, but also with discussion/research to help guide us to the best possible approach :)

I'm not saying you have to do that, but your input would definitely be deeply valued!