Improve Proxy invalid export error message (#84887)
Improve invalid Proxy/Middleware export error message to:
```
> Build error occurred
Error: The file "./proxy.ts" must export a function, either as a default export or as a named "proxy" export.
This function is what Next.js runs for every request handled by this proxy (previously called middleware).
Why this happens:
- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.
- The file exists but doesn't export a function.
- The export is not a function (e.g., an object or constant).
- There's a syntax error preventing the export from being recognized.
To fix it:
- Ensure this file has either a default or "proxy" function export.
Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
```
Turbopack:
```
Error: Turbopack build failed with 1 errors:
./proxy.ts
Proxy is missing expected function export name
This function is what Next.js runs for every request handled by this proxy (previously called middleware).
Why this happens:
- You are migrating from `middleware` to `proxy`, but haven't updated the exported function.
- The file exists but doesn't export a function.
- The export is not a function (e.g., an object or constant).
- There's a syntax error preventing the export from being recognized.
To fix it:
- Ensure this file has either a default or "proxy" function export.
Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
```
Closes NEXT-4741
