Update usage samples doc with sample of subscribing error event (#164)

* Update usage samples doc with sample of subscribing error event

* Update events.md

* update notes to usage sample error handling

docs/events.md

@@ -1,5 +1,7 @@
 # API Documentation
 
+If this is your first time using the Shell-SDK, then we recommend to have a look on our [usage samples](./usage-sample.md).
+
 ## Events
 
 - ### REQUIRE_CONTEXT

docs/usage-sample.md

@@ -90,6 +90,37 @@
     Shell->>Application: SHELL_EVENTS.Version1.REQUIRE_CONTEXT
   ```
 
+- ## Subscribing to errors coming from shell host application
+
+  We highly recommend to include this event to your application, because it is the main way to catch errors communicated from the Shell.
+
+  ```typescript
+  shellSdk.emit(SHELL_EVENTS.Version1.REQUIRE_CONTEXT, {
+    clientIdentifier: '<your-app-client-identifier>',
+    clientSecret: '<your-app-client-secret>',
+  });
+
+  shellSdk.on(SHELL_EVENTS.Version1.REQUIRE_CONTEXT, handler);
+  shellSdk.on(SHELL_EVENTS.ERROR, errorhandler);
+
+  const handler = (context, origin) => {
+    // Will be not triggered in case of an error
+  };
+
+  const errorhandler = (error) => {
+    // Error with information of the failed event sent to Shell
+  };
+  ```
+
+  A proper error handling should include :
+
+  - User notification that an error happened.
+  - Wait time before the same event (error causing) is send again to Shell.
+  - Limit amount of retry. When the same error happens again and again, don't continue to call the event in an infinity loop.
+
+  Missing proper error handling can result in too many (misuse) API calls.
+  In worst case such a violation can result in deactivation of the extension.
+
 - ## Unsubscribing from event
 
   to unsibscribe use _off_ method from _ShellSdk_