Improve plugin loading error message

Summary:
I was a dummy and ran into this. Tried to make it easier
for the next person to figure out what this means.

Reviewed By: mweststrate

Differential Revision: D28026376

fbshipit-source-id: ad3455938ef30b448b475cb990f86351e76628bf

desktop/app/src/dispatcher/plugins.tsx

@@ -289,8 +289,16 @@ const requirePluginInternal = (
     if (plugin.default) {
       plugin = plugin.default;
     }
-    if (!(plugin.prototype instanceof FlipperBasePlugin)) {
-      throw new Error(`Plugin ${plugin.name} is not a FlipperBasePlugin`);
+    if (plugin.prototype === undefined) {
+      throw new Error(
+        `Plugin ${pluginDetails.name} is neither a class-based plugin nor a Sandy-based one.
+        Ensure that it exports either a FlipperPlugin class or has flipper-plugin declared as a peer-dependency and exports a plugin and Component.
+        See https://fbflipper.com/docs/extending/sandy-migration/ for more information.`,
+      );
+    } else if (!(plugin.prototype instanceof FlipperBasePlugin)) {
+      throw new Error(
+        `Plugin ${pluginDetails.name} is not a FlipperBasePlugin`,
+      );
     }
 
     if (plugin.id && pluginDetails.id !== plugin.id) {