message: give a useful error message when rustwide fails to initialize
diff: @@ -161,7 +161,7 @@ pub fn main() {
  docbuilder.save_cache().expect("Failed to save cache");
  } else if let Some(matches) = matches.subcommand_matches("crate") {
  docbuilder.load_cache().expect("Failed to load cache");
  - let mut builder = RustwideBuilder::init().unwrap();
  + let mut builder = RustwideBuilder::init().expect("failed to initialize rustwide");
  match matches.value_of("LOCAL") {
  Some(path) => builder.build_local_package(&mut docbuilder, Path::new(path)),
  None => builder.build_package(&mut docbuilder,
  