Provide output for all our process::Command calls

[tombh]

It would be good to capture STDERR like this String::from_utf8_lossy(&output.stderr) for all shell calls, so we can provide it to the user in the event of a failure. Currently, without this extra information, there's no way to debug the various cargo commands that are called before the actual shader build.

An example of the current problem is that I compiled a shader on a new user account that didn't have the correct Rust toolchain for the shader's workspace. That meant that the call to cargo tree (to query the spirv-std version) failed. But it didn't provide the error, so I had to manually run cargo tree to see what was going.

[schell]

I think we could probably write an extension to Command that does this for us and then use that at all the callsites?