fix: improve error message on incorrect service export (#6240)

**What**
- Provide a more helpful error message if a user adds a file in `src/services` that doesn't export a service class.

**Why**
If you forget to `export default MyClass` in a custom service you can end up with this error:
![CleanShot 2024-01-27 at 6  24 19](https://github.com/medusajs/medusa/assets/7554214/d3700487-33e0-4629-b2c4-a2a37cb86b3e)

It's almost impossible to know how to recover from this which can be an issue for new users. The new error message informs the user that there is a missing class export and shows which file the error is related to.

.changeset/smart-ants-compete.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): improve error message on incorrect service export

packages/medusa/src/loaders/plugins.ts

@@ -469,6 +469,12 @@ export async function registerServices(
       const loaded = require(fn).default
       const name = formatRegistrationName(fn)
 
+      if (typeof loaded !== "function") {
+        throw new Error(
+          `Cannot register ${name}. Make sure to default export a service class in ${fn}`
+        )
+      }
+
       const context = { container, pluginDetails, registrationName: name }
 
       registerPaymentProcessorFromClass(loaded, context)