io error message in `make_tests`

[colin-kiegel]

This would have helped me to understand a broken configuration more quickly. I had compiletest set up to look for a couple of directories, but one was missing and it took me a while to figure it out.

PS: The previous error message was generic, something like "Path does not exist".

[laumann]

Hey @colin-kiegel, sorry for not getting back on this PR sooner.

I'm somewhat hesitant to merge a change like this, because this tool is meant to just be an extraction of the same tool in rust-lang/rust. The codebase here hasn't kept up with rust-lang/rust in a while (primarily because no one has asked), but if that happens, then a change like this is likely to get lost.

On the other hand, this is pretty trivial, so I'm thinking to merge it and that'll be that. May I suggest you submit this as a PR for rust-lang/rust as well?

[colin-kiegel]

Yes, that's ok for me.