When error happens, show full path of found `_quarto.yml`

[cscheid]

The confusion in #6625 would have been avoided if Quarto showed the path to the detected project.

[cscheid]

Fixed by 54bbd97 (no tests because we don't have good infrastructure for testing/regressing on error messages)

[cderv]

(no tests because we don't have good infrastructure for testing/regressing on error messages)

I think we have what is required to test it. I added a test for this in

• Add test for error message regarding validation problem on _quarto.yml #11244

That way it is covered