could handle 300-level responses better

I have an API endpoint that returns a 303 redirect on success with an empty body. The response contains a header that I want to be able to get at. I'm not sure this is intended to be supported with Progenitor. However, the current failure mode is:

    Unexpected Response: Response { url: Url { scheme: "http", cannot_be_a_base: false, username: "", password: None, host: Some(Ipv4(127.0.0.1)), port: Some(12220), path: "//login/test-suite-silo/local", query: None, fragment: None }, status: 303, headers: {"location": "/", "set-cookie": "session=cdb10d5430e901b00050ff41480ad0afdc6869ee; Path=/; HttpOnly; SameSite=Lax; Max-Age=28800", "x-request-id": "6a543c5a-bc48-43ac-8cb2-b0352d08b550", "content-length": "0", "date": "Tue, 02 May 2023 00:00:53 GMT"} }

Here's the spec for this endpoint:

    "/login/{silo_name}/local": {
      "post": {
        "tags": [
          "login"
        ],
        "summary": "Authenticate a user via username and password",
        "operationId": "login_local",
        "parameters": [
          {
            "in": "path",
            "name": "silo_name",
            "required": true,
            "schema": {
              "$ref": "#/components/schemas/Name"
            }
          }
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/UsernamePasswordCredentials"
              }
            }
          },
          "required": true
        },
        "responses": {
          "303": {
            "description": "redirect (see other)",
            "headers": {
              "location": {
                "description": "HTTP \"Location\" header",
                "style": "simple",
                "required": true,
                "schema": {
                  "type": "string"
                }
              }
            }
          },
          "4XX": {
            "$ref": "#/components/responses/Error"
          },
          "5XX": {
            "$ref": "#/components/responses/Error"
          }
        }
      }
    },

which shows that it has a 303 response.

It might be fair to say that 300-level responses aren't supported, especially since reqwest follows 300s by default, so in principle Progenitor wouldn't usually see them. In this case, I configured my reqwest client not to follow the redirect so that I could get the header I want back. If that's not supported though then I think we should at least produce a clearer error (e.g., 300-level response codes are not supported). We may even want to fail to generate a client for a spec with a 300-level response? Or emit a warning and skip that endpoint altogether?

@ahl noticed we don't handle 300-level responses here:
https://github.com/oxidecomputer/oxide.rs/blob/d007f734e8f379d8a174d45c18aabd67ec24eef5/sdk/src/generated_sdk.rs#L20053-L20062

Gory details (which I hope aren't necessary, but if you want to repro this): https://github.com/davepacheco/oxide-client-test

Start with sdk/src/generated_sdk.rs around the linked response-handling code and inspect how Progenitor treats non-2xx responses. Reproduce the 303 case using the linked oxide-client-test details or an equivalent reqwest client configuration. The issue is resolved when 300-level responses are either supported for accessing headers or rejected with a clear, tested error, according to a decided behavior.