We consider poor error messages to be bugs. I know most people don't, so I've made it one of my missions to yell it from the mountain tops to encourage people to report more often. We can't fix what we don't know about. The majority of the current good errors were a reaction to things people filed in the past.
Thanks for looking into it. I have a filed a bug against Rust before, but that was a clear bug, not a poor error message. Remember, the only time a user sees something like this is when they are trying to do something else. The only reason I looked into it just now is because you explicitly asked.