Give helpful info with r? is invalid

[jackh726]

Right now, if someone does r? compiler, nothing happens. It would be helpful to have a comment added along the lines of something like:

Error! That reviewer/group does not exist. Did you mean one of `rust-lang/compiler`, `rust-lang/compiler-team`, or  `rust-lang/compiler-team-contributors`?

You can view a full list of available reviews [here](https://github.com/rust-lang/highfive/blob/master/highfive/configs/rust-lang/rust.json).

Also, it would be nice if suggested "close" suggestions; so, for example, doing r? rust-lang/wg-traits would give a suggestion for r? rust-lang/traits.

[jyn514]

cc #300