Apply/test custom req_error() handlers

[nealrichardson]

The mock code path in req_perform() returns before applying any req_error() logic. This means that under the default behavior of httr2, a request that returns a 4xx response will error, but capture_requests() will record the response, and on replay in with_mock_api(), the 4xx response will be loaded and returned without error. It also means that any custom error handling a user/package may define won't get tested by mocks.