message: Add error message for missing CRATESFYI_PREFIX
diff: @@ -112,7 +112,7 @@ impl CratesfyiHandler {
  let shared_resources = Self::chain(&pool_factory, rustdoc::SharedResourceHandler);
  let router_chain = Self::chain(&pool_factory, routes.iron_router());
  - let prefix = PathBuf::from(env::var("CRATESFYI_PREFIX").unwrap()).join("public_html");
  + let prefix = PathBuf::from(env::var("CRATESFYI_PREFIX").expect("the CRATESFYI_PREFIX environment variable is not set")).join("public_html");
  let static_handler = Static::new(prefix)
  .cache(Duration::from_secs(STATIC_FILE_CACHE_DURATION));
  