cargo verify-project returns incorrect results when using unstable features

[emilyalbini]

cargo verify-project returns a positive response if a crate is using unstable features on the stable toolchain, even if every other cargo command returns an error.

$ cargo read-manifest
error: failed to parse manifest at `/.../Cargo.toml`

Caused by:
  the cargo feature `test-dummy-unstable` requires a nightly version of Cargo, but this is the `stable` channel

$ cargo verify-project
{"success":"true"}

Sample Cargo.toml

cargo-features = ["test-dummy-unstable"]

[package]
name = "foo"
version = "0.1.0"
authors = ["Pietro Albini <pietro@pietroalbini.org>"]

[dependencies]