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

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