Error reason not propagating on some status codes

[ealsur]

Description

When executing a server requests, we have 2 possible errors:

• Errors that generate exceptions
• Errors that return the response directly (409, 412, 404)

For the first one, we convert the exception into a ResponseMessage with the ErrorMessage property coming from the exception.

The second kind gets treated as a normal response and passed forward, but the actual reason for the error is within the ResponseBody.

This PR fixes this scenario by making sure that, when EnsureSuccessStatusCode is called on a ResponseMessage, we generate the ErrorMessage based on the body if it wasn't already set. So for the cases where the errors are coming from exceptions, this is a no-op, but for the cases of errors coming as responses, we materialize the body and obtain the error.

Additionally, as part of the investigation, a bug was found in the GatewayStoreClient that was not propagating the body in the case of such status codes.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Closing issues

Closes #681