Added errors store with tests

frontend/stores/ErrorsStore.js

@@ -0,0 +1,18 @@
+// @flow
+import { observable, action } from 'mobx';
+
+class UiStore {
+  @observable errors = observable.array([]);
+
+  /* Actions */
+
+  @action add = (errorMessage: string): void => {
+    this.errors.push(errorMessage);
+  };
+
+  @action remove = (index: number): void => {
+    this.errors.splice(index, 1);
+  };
+}
+
+export default UiStore;