message: better error when using cargo-miri in a workspace
diff: @@ -91,7 +91,7 @@ fn list_targets() -> impl Iterator<Item=cargo_metadata::Target> {
  let mut metadata = if let Ok(metadata) = cmd.exec() {
  metadata
  } else {
  - show_error(format!("Could not obtain Cargo metadata"));
  + show_error(format!("Could not obtain Cargo metadata; likely an ill-formed manifest"));
  };
  let current_dir = std::env::current_dir();
  @@ -113,7 +113,7 @@ fn list_targets() -> impl Iterator<Item=cargo_metadata::Target> {
  package_manifest_directory == current_dir
  }
  })
  - .expect("could not find matching package");
  + .unwrap_or_else(|| show_error(format!("This seems to be a workspace, which is not supported by cargo-miri")));
  let package = metadata.packages.remove(package_index);
  // Finally we got the list of targets to build
  