Added errors store with tests

.flowconfig

@@ -6,6 +6,7 @@
 .*/node_modules/react-side-effect/.*
 .*/node_modules/fbjs/.*
 .*/node_modules/config-chain/.*
+*.test.js
 
 [libs]
 

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;

frontend/stores/ErrorsStore.test.js

@@ -0,0 +1,27 @@
+/* eslint-disable */
+import ErrorsStore from './ErrorsStore';
+
+// Actions
+describe('ErrorsStore', () => {
+  let store;
+
+  beforeEach(() => {
+    store = new ErrorsStore();
+  });
+
+  test('#add should add errors', () => {
+    expect(store.errors.length).toBe(0);
+    store.add('first error');
+    store.add('second error');
+    expect(store.errors.length).toBe(2);
+  });
+
+  test('#remove should remove errors', () => {
+    store.add('first error');
+    store.add('second error');
+    expect(store.errors.length).toBe(2);
+    store.remove(0);
+    expect(store.errors.length).toBe(1);
+    expect(store.errors[0]).toBe('second error');
+  });
+});