Be clearer about which repo failed to be fetched

This is a terrible patch, and I know it. But it'll get the conversation started. This issue is related to https://github.com/Homebrew/homebrew/issues/24416, in the sense that in that case there was also a dissapearing repo. In that case, it was an official one and it was resolved by no longer having that tap included by default. In my case, I added someone's tap myself, but that user deleted his/her repository, and now `brew search foo` is failing for me, because it can't find *some* tap. However, it doesn't tell me which tap that is.

This patch improves that in the sense that it now tells the user which tap has problems. It also makes it worse since it's no longer clear what the specific HTTP error is. 

I'm more than willing to persue this further. But I wanted to start this discussion early. For instance, it might make sense to also add checks for the reachability of taps to `brew doctor`. We might want to have a system for warning people about issues while searching, without halting the program. So, how far down the rabbit hole should I go? Are there any automated tests? I couldn't find any. Are you interested in having tests? I'd be willing to start up some foundations, so that in the future it's easier to expand on tests.