(core) Error message on Duplicate Document

Summary: Fixing error message when user can't copy document.

Test Plan: Updated tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3130

app/client/models/NotifyModel.ts

@@ -304,6 +304,7 @@ export class Notifier extends Disposable implements INotifier {
   // This is exposed primarily for tests.
   public clearAppErrors() {
     this._appErrorList.splice(0);
+    this._appErrorToast.clear();
   }
 
   /**