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

This covers a particular case of #946.

This commit is actually NFC as far as the interaction with the server
goes; its only change in behavior is that we treat the error as
"expected", and therefore skip reporting it to the user unless it
follows a string of other errors.

But then this also sets us up so that these rate-limit errors will
continue to be handled with backoff-retry when we change the handling
of unexpected errors, coming next.

Author: gnprice

lib/model/store.dart

@@ -1054,6 +1054,13 @@ class UpdateMachine {
             isUnexpected = false;
             shouldReportToUser = true;
 
+          case ServerException(httpStatus: 429):
+          case ZulipApiException(httpStatus: 429):
+          case ZulipApiException(code: 'RATE_LIMIT_HIT'):
+            // TODO(#946) handle rate-limit errors more generally, in ApiConnection
+            isUnexpected = false;
+            shouldReportToUser = true;
+
           default:
             isUnexpected = true;
             shouldReportToUser = true;

test/model/store_test.dart

@@ -919,19 +919,19 @@ void main() {
       });
 
       test('report rate limit: code RATE_LIMIT_HIT', () {
-        checkReported(prepareRateLimitExceptionCode).startsWith(
+        checkLateReported(prepareRateLimitExceptionCode).startsWith(
           "Error connecting to Zulip. Retrying…\n"
           "Error connecting to Zulip at");
       });
 
       test('report rate limit: status 429 ZulipApiException', () {
-        checkReported(prepareRateLimitExceptionStatus).startsWith(
+        checkLateReported(prepareRateLimitExceptionStatus).startsWith(
           "Error connecting to Zulip. Retrying…\n"
           "Error connecting to Zulip at");
       });
 
       test('report rate limit: status 429 MalformedServerResponseException', () {
-        checkReported(prepareRateLimitExceptionMalformed).startsWith(
+        checkLateReported(prepareRateLimitExceptionMalformed).startsWith(
           "Error connecting to Zulip. Retrying…\n"
           "Error connecting to Zulip at");
       });