Show correct file name in middleware error message

[rebase]

Goals

Display which file is detected as middleware in error messages

Non-Goals

Background

When output: 'export'(for static exports) is configured, the build fails with the error:
⨯ Middleware cannot be used with "output: export".

The problem is that the error message doesn't specify which file is causing the conflict. In my case, I was using proxy.ts (based on https://nextjs.org/docs/app/api-reference/file-conventions/proxy) rather than middleware.ts. Next.js provided no clue in the logs as to which file triggered the error, making it difficult to debug.

While I was able to identify that proxy.ts was being treated as middleware since I changed the file name myself, this might be very confusing for people who are new to the framework.

Proposal

⨯ Middleware in 'proxy.ts' cannot be used with "output: export"

Not sure but maybe this kind of simple change would save developers time by immediately showing which file needs to be fixed.