When:
1. the proxy is autodetected from `http_proxy` etc. variables;
2. a request is made which bypasses the proxy (e.g. to an authority in `no_proxy`);
3. this request fails with one of `Proxy::PROXY_ERRORS` (timeout or a system error)
the `fetch_response` method tried to access the proxy URIs array which
isn't initialized by `proxy_options`. This change fixes the
`proxy_error?` check to avoid the issue.
