message: Better expect messages
diff: @@ -313,7 +313,10 @@ fn resolve_dependency(
  selected
  } else {
  let source = crate::sources::PathSource::new(&path, src.source_id()?, config);
  - let package = source.read_packages()?.pop().expect("at least one package");
  + let package = source
  + .read_packages()?
  + .pop()
  + .expect("read_packages errors when no packages");
  Dependency::from(package.summary())
  };
  selected
  @@ -603,7 +606,10 @@ fn infer_package_for_git_source(
  src: &dyn std::fmt::Display,
  ) -> CargoResult<Package> {
  let package = match packages.len() {
  - 0 => unreachable!(),
  + 0 => unreachable!(
  + "this function should only be called with packages from `GitSource::read_packages` \
  + and that call should error for us when there are no packages"
  + ),
  1 => packages.pop().expect("match ensured element is present"),
  _ => {
  let mut names: Vec<_> = packages
  