control/controlclient: include HTTP status string in error message too

Not just its code.

Updates tailscale/corp#23584

Change-Id: I8001a675372fe15da797adde22f04488d8683448
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -1229,7 +1229,7 @@ func loadServerPubKeys(ctx context.Context, httpc *http.Client, serverURL string
 		return nil, fmt.Errorf("fetch control key response: %v", err)
 	}
 	if res.StatusCode != 200 {
-		return nil, fmt.Errorf("fetch control key: %d", res.StatusCode)
+		return nil, fmt.Errorf("fetch control key: %v", res.Status)
 	}
 	var out tailcfg.OverTLSPublicKeyResponse
 	jsonErr := json.Unmarshal(b, &out)