leave non-500 errors alone

src/app_error.rs

···
         let is_dev = env::var("ENVIRONMENT").unwrap_or("development".to_string()) == "development";
         let body = if status_code == StatusCode::INTERNAL_SERVER_ERROR && !is_dev {
             "internal server error".to_string()
-        } else {
+        } else if status_code == StatusCode::INTERNAL_SERVER_ERROR {
             let body = if let Self::Anyhow(e) = &self
                 && let Some(template_error) = e.downcast_ref::<minijinja::Error>()
             {
···
             };
 
             render_error(body)
+        } else {
+            self.to_string()
         };
 
         (self.status_code(), Html(body)).into_response()