store [nfc]: Add account-exists assert in _reloadPerAccount

I believe this is effectively already done by the existing assert
`_perAccountStores.containsKey`, because of the invariant that
_accounts contains all the keys in _perAccountStores. But the
explicitness seems helpful. (And loadPerAccount relies on the
account existing.)

Author: chrisbobbe

lib/model/store.dart

@@ -154,6 +154,7 @@ abstract class GlobalStore extends ChangeNotifier {
   }
 
   Future<void> _reloadPerAccount(int accountId) async {
+    assert(_accounts.containsKey(accountId));
     assert(_perAccountStores.containsKey(accountId));
     assert(!_perAccountStoresLoading.containsKey(accountId));
     final store = await loadPerAccount(accountId);