message: feat: ensure main file exists
  If the file doesn't exists show an error to the user.
diff: @@ -14,11 +14,18 @@ impl Package {
  failure::bail!(
  "The `main` key in your `package.json` file is required; please specified the entrypoint of your Worker.",
  )
  + } else {
  + if !Path::new(&self.main).exists() {
  + failure::bail!(
  + "The entrypoint of your Worker ({}) could not be found.",
  + self.main
  + )
  } else {
  Ok(self.main.clone())
  }
  }
  }
  +}
  impl Package {
  pub fn new(pkg_path: &str) -> Result<Package, failure::Error> {
  