API: Update schema "required" properties

To ensure API responses are consistent. Also most of these properties
are expected from a consumer point of view, even if null.

Mark Error schema properties nullable where not in required list.

tildes/openapi_beta.yaml

@@ -345,6 +345,9 @@ components:
         - exemplary
         - collapsed
         - collapsed_individual
+        - by_op
+        - by_me
+        - new_comment
         - voted
         - bookmarked
         - depth
@@ -429,9 +432,11 @@ components:
       required:
         - message
       properties:
-        field:
-          type: string
         message:
           type: string
+        field:
+          type: string
+          nullable: true
         exception:
           type: string
+          nullable: true