Executor returns minimal information on http client failure

[mwm5945]

Describe the bug

When the executor HTTP client experiences an error (EX, a timeout), it only returns a 500 code header, with an empty body.

To reproduce

1. Deploy model that takes a while (say 1s) with a very short timeout (say 500ms)
2. Ensure Executor is enabled for deployment
3. query the prediction endpoint
4. observe the empty body and 500 response code.

Expected behaviour

A clearer response is returned from the executor, including the error from the executor.