Hm. I'm not sure if any sort of warning is necessary, rather than just traditional. But I think it probably is, if only to let you react in a socially appropriate way, rather than say "what, did you really say that?"

I think giving a sort of generalised warning is reasonable, and probably prefer the second of your options, but that's because what I really, really want to avoid is suggesting that the news might be worse than it is.

"I'm afraid I've got something to tell you" is just awful, because it makes you immediately imagine ALL the possible worse scenarios, and (I don't know if other people agree but) I really hate being momentarily terrified to no good purpose.

I'd prefer the warning clearly bounded above how bad the news way. If a close friend or relative hasn't died or been deported, you don't really want to spend much time imagining that they have, and if they HAVE you don't really want to spend much time worrying about the other one either.

And you REALLY REALLY don't want to spend that much emotional investment if someone wants you to make a minor, sensible and inconsequential change to a piece of software.