Convert Add Movie from collection to TypeScript

frontend/src/App/State/MovieCollectionAppState.ts

@@ -1,6 +1,7 @@
 import AppSectionState, {
   AppSectionFilterState,
   AppSectionSaveState,
+  Error,
 } from 'App/State/AppSectionState';
 import MovieCollection from 'typings/MovieCollection';
 
@@ -10,6 +11,9 @@ interface MovieCollectionAppState
     AppSectionSaveState {
   itemMap: Record<number, number>;
 
+  isAdding: boolean;
+  addError: Error;
+
   pendingChanges: Partial<MovieCollection>;
 }