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

@@ -1065,8 +1065,7 @@ 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;