message: Improve error message on missing feature gate
diff: @@ -520,7 +520,7 @@ impl Manifest {
  .require(Feature::per_package_target())
  .chain_err(|| {
  anyhow::format_err!(
  - "the `package.default-kind` and `package.forced-kind` \
  + "the `package.default-target` and `package.forced-target` \
  manifest keys are unstable and may not work properly"
  )
  })?;
  