message: more helpful error on workspaces
diff: @@ -144,7 +144,8 @@ fn list_targets() -> impl Iterator<Item = cargo_metadata::Target> {
  })
  .unwrap_or_else(|| {
  show_error(format!(
  - "This seems to be a workspace, which is not supported by cargo-miri"
  + "this seems to be a workspace, which is not supported by `cargo miri`.\n\
  + Try to `cd` into the crate you want to test, and re-run `cargo miri` there."
  ))
  });
  let package = metadata.packages.remove(package_index);
  