Start abstractions for the guide

Summary:
This is the third diff for M3

- Started abstractions to remove repetitive code as a part of M3
- Implemented a new abstracted component in GuideAnserScreen and removed PluginErrored.tsx to integrate it here.
- Using a map object that maps a string to function. Here I have used helper functions to get each screens context as first class members.
- The map is global and is populated globally (as discussed) and we invoke the required function based on the state we are at from GuideAnswerScreen.

Reviewed By: mweststrate

Differential Revision: D30772236

fbshipit-source-id: f8fc175c6d308e2f72c5f6ce826f70245ad46797

desktop/app/src/utils/assertNotNull.tsx

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+export function assertNotNull<T extends any>(
+  value: T,
+  message: string = 'Unexpected null/undefined value found',
+): asserts value is Exclude<T, undefined | null> {
+  if (value === null || value === undefined) {
+    throw new Error(message);
+  }
+}