Pass ProxyPrefix into the error template.

The default `error.html` uses `ProxyPrefix` but it isn't supplied in the context, causing it to error.

oauthproxy.go

@@ -289,9 +289,11 @@ func (p *OauthProxy) ErrorPage(rw http.ResponseWriter, code int, title string, m
 	t := struct {
 		Title       string
 		Message     string
+		ProxyPrefix string
 	}{
 		Title:       fmt.Sprintf("%d %s", code, title),
 		Message:     message,
+		ProxyPrefix: p.ProxyPrefix,
 	}
 	p.templates.ExecuteTemplate(rw, "error.html", t)
 }