Return a meaningful errors if agent cannot connect upstream host

[pablochacin]

Presently, when the agent gets an error connecting to the upstream host (the one it has to forward for), it returns an internal server error.

This may be confusing, as the internal server is a common fault to inject in a test. Actually, it happens in e2e tests that inject this error and the pass even if the agent fails to connect the upstream host because it is returning this error, instead of reporting it had an issue completing the request.

Therefore, it would be convenient that in this situation the agent returns a meaningful error such as 'bad gateway' to indicate there was an issue connecting to the upstream service.