store [nfc]: Use try/finally to clear _perAccountStoresLoading[accountId]

In GlobalStore.perAccount, the loadPerAccount future can throw, and
in that case the `_perAccountStoresLoading.remove` in these lines of
code hasn't been getting called.

That's only a latent bug, currently, because loadPerAccount only
throws if the account was logged out, and we always do a
`_perAccountStoresLoading.remove` on logout (in
GlobalStore.removeAccount).

But we'd like to add another case where the loadPerAccount future
throws (disallow connecting to ancient servers), which doesn't
involve logout / removeAccount. So this try/finally will be
important for that case.

Author: chrisbobbe

lib/model/store.dart

@@ -124,10 +124,13 @@ abstract class GlobalStore extends ChangeNotifier {
     // It's up to us.  Start loading.
     future = loadPerAccount(accountId);
     _perAccountStoresLoading[accountId] = future;
-    store = await future;
-    _setPerAccount(accountId, store);
-    unawaited(_perAccountStoresLoading.remove(accountId));
-    return store;
+    try {
+      store = await future;
+      _setPerAccount(accountId, store);
+      return store;
+    } finally {
+      unawaited(_perAccountStoresLoading.remove(accountId));
+    }
   }
 
   Future<void> _reloadPerAccount(int accountId) async {