Improve bug message in check_ty
Improve bug message in check_ty

This branch was hit in Clippy and I think it would be nice to
show the thing that was unexpected in the bug message.

It's also in line with the other `bug!` messages in `check_ty`.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust