build-kit: improve error message when `cargo` and `cargo-make` are not installed

[sam-berning]

If you try to run twoliter build kit when cargo and cargo-make are not installed, you will just get an error message like

Error: Unable to start command

with no hint as to why it was unable to start.

This may affect other twoliter build commands as well, but I haven't confirmed that yet.

We should either surface the stderr of the command here or explicitly check if cargo and cargo-make exist.