WIP report retry of register, via SnackBar

This works fine, but UX seems not good -- SnackBar is way too busy
given this can retry again and again, and can be perfectly normal
if you're simply not connected.

Author: gnprice

lib/model/store.dart

@@ -730,7 +730,8 @@ class UpdateMachine {
       } catch (e) {
         assert(debugLog('Error fetching initial snapshot: $e\n'
           'Backing off, then will retry…'));
-        // TODO tell user if initial-fetch errors persist, or look non-transient
+        // TODO perhaps suppress this message if error transient
+        reportErrorToUserBriefly('Error loading server data, will retry: $e');
         await (backoffMachine ??= BackoffMachine()).wait();
         assert(debugLog('… Backoff wait complete, retrying initial fetch.'));
       }