store [nfc]: Use HttpException to simplify error cases

This expresses more directly what we really mean here: if the
response had either the HTTP status code or the Zulip API error code
that mean a rate-limit error, treat it as a rate-limit error.

Author: gnprice

lib/model/store.dart

@@ -1128,8 +1128,7 @@ class UpdateMachine {
       case Server5xxException():
         shouldReportToUser = true;
 
-      case ServerException(httpStatus: 429):
-      case ZulipApiException(httpStatus: 429):
+      case HttpException(httpStatus: 429):
       case ZulipApiException(code: 'RATE_LIMIT_HIT'):
         // TODO(#946) handle rate-limit errors more generally, in ApiConnection
         shouldReportToUser = true;