Print an error message for unsupported cargo completions
Print an error message for unsupported cargo completions

inejge/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs