I take slight issue with these two points:
Bottom line - there too many unnecessary/erroneous down votes.
The up voting on the other hand has a opposite problem and doesn't need confirmation in my opinion as it should be encouraged. Often times the up votes are severely lacking even though they should exist.
Up-voting and down-voting both take the same amount of effort. To say that it's easy to accidentally click the down-vote button and forget about it is to concede the argument that it is similarly easy to click the up-vote button and forget about it. Down-voting is even gated behind a low reputation requirement, which implies that less people overall can perform the action. To say that there are too many erroneous down-votes is to also ignore that, as rene points out, there are far more up-votes across Stack Overflow than down-votes. Feel free to manipulate that query, and you'll see that this trend is pretty much the same across other sites in the network.
So... Up-votes do exist, and they're far more prevalent than down-votes. As a result, I don't believe it's necessary to put an intermediary step between a user clicking a button that indicates how useful posts are, and that vote taking effect. Voting, both up and down, is important! Moderating content through votes has always been easy to do, and interrupting that flow with an intermediary step seems unnecessary at best and counter-productive to content moderation at worst.
If I went to the trouble of pressing the down-vote button on a post I felt wasn't useful, then I'd simply press "Yes, I'm sure" on the prompt you suggest. Without your suggestion, if I accidentally clicked the down-vote button, I'd probably notice that the button is now highlighted, and (as Glorfindel points out) I can simply undo my vote. Sure, your suggestion may have saved me the trouble of clicking the button again, but it wouldn't stop me from down-voting a post I feel isn't useful.