cargo install should tell the available version when already installed

[birkenfeld]

Currently, when trying to install an already-installed crate, we get:

$ cargo install rustfmt
    Updating registry `https://github.com/rust-lang/crates.io-index`
error: binary `cargo-fmt` already exists in destination as part of `rustfmt v0.4.1`

This tells me the currently installed version of rustfmt, but not which version it would be replaced with when I specify -f. This would be interesting especially to know if there is an updated version at all.

When the crates cache is not populated with the new version, we get

$ cargo install rustfmt
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading rustfmt v0.7.1
error: binary `cargo-fmt` already exists in destination as part of `rustfmt v0.4.1`

which is helpful, but accidental.