Adjust the error messages to match the pattern "expected foo, found bar"

Closes #8492
auto merge of #16699 : treeman/rust/issue-8492, r=alexcrichton

Closes #8492.

I did not find this suggestion in the [guidelines][] but it's mentioned in the [old style guide][].

[guidelines]: https://github.com/rust-lang/rust-guidelines
[old style guide]: https://github.com/rust-lang/rust/wiki/Note-style-guide/73c864a10a8e231e2a6630e5a3461f1d3022a20a