[TypeSpec Requirement] Improve error logging to provide more information directly in PR

I can't remember exactly why I opened this.

If it was just for adding text to "next steps to merge", then it's a dupe of https://github.com/Azure/azure-sdk-tools/issues/7542.

But it might have been to improve the text in one or both of these error messages, which is unrelated:

```
##[error]New specs must use TypeSpec. For more detailed docs see https://aka.ms/azsdk/typespec

##[error]specification/foo/resource-manager/foo/preview/2023-11-01-preview/foo.json(1,1):
error: OpenAPI was not generated from TypeSpec, and spec appears to be new
```