store [nfc]: Pull out _clearReportingErrorsToUser from poll method

This logic pairs closely with _maybeReportToUserTransientError,
so pulling it out lets that logic live nearer each other.

Author: gnprice

lib/model/store.dart

@@ -1094,9 +1094,7 @@ class UpdateMachine {
         backoffMachine = null;
 
         store.isLoading = false;
-        // Dismiss existing errors, if any.
-        reportErrorToUserBriefly(null);
-        _accumulatedTransientFailureCount = 0;
+        _clearReportingErrorsToUser();
 
         final events = result.events;
         for (final event in events) {
@@ -1182,6 +1180,11 @@ class UpdateMachine {
 
   int _accumulatedTransientFailureCount = 0;
 
+  void _clearReportingErrorsToUser() {
+    _accumulatedTransientFailureCount = 0;
+    reportErrorToUserBriefly(null);
+  }
+
   /// This only reports transient errors after reaching
   /// a pre-defined threshold of retries.
   void _maybeReportToUserTransientError(Object error) {