Adding 501 not implemented errors to screen API which did not have a controller previously.

packages/server/src/api/controllers/screen.js

@@ -0,0 +1,17 @@
+/**
+ * This controller is not currently fully implemented. Screens are
+ * currently managed as part of the pages API, please look in api/routes/page.js
+ * for routes and controllers.
+ */
+
+exports.fetch = async ctx => {
+  ctx.throw(501)
+}
+
+exports.save = async ctx => {
+  ctx.throw(501)
+}
+
+exports.destroy = async ctx => {
+  ctx.throw(501)
+}