fix: name the HTTP status when the error body is not JSON

errors_test.go

@@ -252,9 +252,12 @@ func TestBuildAPIError_UnparseableBodyPath(t *testing.T) {
 	if got.Code != 0 {
 		t.Errorf("Code = %d, want 0 for unparseable", got.Code)
 	}
-	if got.Message != "failed to parse error response" {
+	if got.Message != "failed to parse error response: unexpected server response code 500" {
 		t.Errorf("Message = %q, want canonical unparseable text", got.Message)
 	}
+	if got.StatusCode != http.StatusInternalServerError {
+		t.Errorf("StatusCode = %d, want the transport status preserved", got.StatusCode)
+	}
 	if got.RawResponseBody != body {
 		t.Errorf("RawResponseBody = %q, want raw body preserved", got.RawResponseBody)
 	}

http.go

@@ -131,8 +131,11 @@ func buildAPIError(resp *http.Response, body []byte) *StreamError {
 
 	if len(body) > 0 {
 		if err := json.Unmarshal(body, apiErr); err != nil {
+			// A failed Unmarshal can still have assigned fields before it errored,
+			// so restore the transport status rather than trust the partial parse.
+			apiErr.StatusCode = resp.StatusCode
 			apiErr.Code = 0
-			apiErr.Message = "failed to parse error response"
+			apiErr.Message = fmt.Sprintf("failed to parse error response: unexpected server response code %d", resp.StatusCode)
 			apiErr.ExceptionFields = map[string]string{}
 			apiErr.Unrecoverable = false
 			apiErr.cause = stackWrap(err, "parse api error response")