fixes #925 add topics to simple linked list exercise (#933)

* #925 add topics to simple linked list exercise

* added exception_handling to topics for simple linked list

config.json

       "core": false,
       "difficulty": 7,
       "slug": "simple-linked-list",
-      "topics": null,
+      "topics": [
+        "algorithms",
+        "generics",
+        "lists",
+        "exception_handling"
+      ],
       "unlocked_by": "linked-list",
       "uuid": "e3e5ffe5-cfc1-467e-a28a-da0302130144"
     },