When http_client.request fails synchronously (e.g. RobotsLayer returning
RobotsBlocked because robots.txt is already cached), Client.request
invokes our error_callback before returning the error. httpErrorCallback
rejects the promise and releases response._arena. Letting the error
propagate from Fetch.init also fires the `errdefer response.deinit`,
double-freeing the arena and corrupting the arena pool — eventually
surfacing as a malloc abort during teardown.
Fixes#2403.
d7e283fed9