Allow installation when /usr/bin/rust is present

$ ./rustup-init
error: it looks like you have an existing installation of Rust at:
error: /usr/bin
error: rustup cannot be installed alongside Rust. Please uninstall first
error: if this is what you want, restart the installation with `-y'
error: cannot install while Rust is installed

This should probably be an interactive question instead, so inexperienced users have a chance to install rustup without removing the systems rust installation. (Which I would consider a bad idea.)