[rustbuild] Should print what command it couldn’t execute

[nagisa]

Rustbuild now prints

failed to execute command: No such file or directory (os error 2)

which is very not helpful.

[vadimcn]

I've hit this one too: When there's a stale config.mk in Rust's root directory, bootstrap reads paths from there and tries to convert them into windows format using cygpath (which it, of course, can't find since we are not running in msys shell).

The error message comes from this line in build_helper (called from here).