message: feat(init): improve error message for init site with existing toml
diff: @@ -10,8 +10,21 @@ pub fn init(
  site_flag: bool,
  ) -> Result<(), failure::Error> {
  if Path::new("./wrangler.toml").exists() {
  + if site_flag {
  + let msg = r#"A wrangler.toml file already exists!
  +
  +To add Workers Sites to your existing wrangler.toml, please add this section:
  +
  +[site]
  +bucket = "" # this should point to the directory with static assets
  +entry-point = "workers-site"
  +
  +"#;
  + failure::bail!(msg);
  + } else {
  failure::bail!("A wrangler.toml file already exists! Please remove it before running this command again.");
  }
  + }
  let dirname = get_current_dirname()?;
  let name = name.unwrap_or_else(|| &dirname);
  validate_worker_name(name)?;
  