message: Write stderr output from build-scripts to disk.
  Closes
diff: @@ -164,7 +164,12 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>)
  let pkg_name = unit.pkg.to_string();
  let build_state = cx.build_state.clone();
  let id = unit.pkg.package_id().clone();
  - let output_file = build_output.parent().unwrap().join("output");
  + let (output_file, err_file) = {
  + let build_output_parent = build_output.parent().unwrap();
  + let output_file = build_output_parent.join("output");
  + let err_file = build_output_parent.join("stderr");
  + (output_file, err_file)
  + };
  let all = (id.clone(), pkg_name.clone(), build_state.clone(),
  output_file.clone());
  let build_scripts = super::load_build_deps(cx, unit);
  @@ -237,7 +242,9 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>)
  pkg_name, e.desc);
  Human(e)
  })?;
  +
  paths::write(&output_file, &output.stdout)?;
  + paths::write(&err_file, &output.stderr)?;
  // After the build command has finished running, we need to be sure to
  // remember all of its output so we can later discover precisely what it
  