docs(error-handling): add originalError property description (#2968)

* docs(error-handling): add originalError property description

* Update errors.md

Author: kabachook

docs/source/features/errors.md

@@ -150,6 +150,20 @@ server.listen().then(({ url }) => {
 });
 ```
 
+The error instance received by `formatError` (a `GraphQLError`) contains an `originalError` property which represents the original error thrown within the resolver.  This can be used to `instanceof` check against a specific error class, such as `AuthenticationError`, `ValidationError`, etc.:
+
+```js
+  /* ... */
+  formatError(err) {
+    if (err.originalError instanceof AuthenticationError) {
+      return new Error('Different authentication error message!');
+    }
+  },
+  /* ... */
+```
+
+> To make context-specific adjustments to the error received by `formatError` (e.g. localization or personalization), consider using the `didEncounterErrors` life-cycle hook to attach additional properties to the error, which can be accessed andn utilized within `formatError`.
+
 ### For Apollo Engine reporting
 
 With the Apollo Platform, it's possible to observe error rates within Apollo