Rollup merge of #140556 - GuillaumeGomez:improve-rustdoc-gui-tool-error-output, r=clubby789

Improve error output in case `nodejs` or `npm` is not installed for rustdoc-gui test suite

Fixes rust-lang/rust#138134.

It now looks like this:

![Screenshot From 2025-05-01 17-41-12](https://github.com/user-attachments/assets/ae7f2b37-3619-47ff-941d-5d9a7cdd9b6b)

cc ``@kpreid``

Author: GuillaumeGomez