wiplink store: On rate-limit error in poll, back off and retry

This covers a particular case of 946.

Author: gnprice

lib/model/store.dart

@@ -977,6 +977,11 @@ class UpdateMachine {
             case Server5xxException():
               shouldReportToUser = true;
 
+            case ZulipApiException(httpStatus: 429):
+            case ZulipApiException(code: 'RATE_LIMIT_HIT'):
+              // TODO(#946) handle rate-limit errors more generally, in ApiConnection
+              shouldReportToUser = true;
+
             default:
               rethrow;
           }