More helpful error for invalid cargo-features = []

[kornelski]

The error message for unknown values of cargo-features was unhelpful:

Caused by:
  unknown cargo feature `build-dir`

I've made it more explanatory, and matching the style of the other errors:

Caused by:
  feature `build-dir` is not a valid unstable feature for Cargo.toml

  This feature can be enabled via -Zbuild-dir or the `[unstable]` section in config.toml.
  See https://doc.rust-lang.org/cargo/reference/unstable.html for more information.

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[epage]

Thanks for working to improve this!

[epage]

Thanks!