message: don't continue if single install errors
diff: @@ -65,7 +65,8 @@ pub fn install(root: Option<&str>,
  let installed_anything = if krates.len() <= 1 {
  install_one(root.clone(), map, krates.into_iter().next(), source_id, vers, opts,
  - force, true).is_ok()
  + force, true)?;
  + true
  } else {
  let mut succeeded = vec![];
  let mut failed = vec![];
  