docs: update onError usage

docs/api/hooks.md

@@ -18,6 +18,13 @@ See [here](/guide/advanced.html#hooks) for usage.
 
 - arguments: `(error)`
 
-If the error originated from a request, the property `err.response` might be available.
+If the error originated from a request.
+
+Available properties:
+
+- `error.statusCode`
+- `error?.response?.data`
+
+You can optionally return a value or promise that can resolve for fallback response. If hook returns any value, other hooks **won't** be called.
 
 See [here](/guide/advanced.html#hooks) for usage.

docs/guide/advanced.md

@@ -42,9 +42,11 @@ export default function ({ $http }) {
   })
 
   $http.onError(error => {
-    if(error.response.status === 500) {
+    if(error.statusCode === 500) {
       alert('Request Error!')
     }
+    // Tip: You can use error.response?.data? to get response message
+    // Tip: You can return another a fallback response (or promise) to avoid rejection
   })
 }
 ```