[rustdoc] Gracefully handle in case we cannot run the compiler in doctests

[GuillaumeGomez]

Fixes bug reported in this comment.

r? @lolbinarycat

[lolbinarycat]

1. I think we should include the path of the file we are trying to execute in the error
2. (related) the message "failed to spawn rustc process" is a bit misleading, since (as far as I understand it) the entire point of this flag is you can pass an executable that isn't rustc, and such an error would likely cause an error to debug issues with rustc, instead of debugging their wrapper.

I think a message like "failed to spawn {compiler_path:?}: {error:?}" would be significantly more clear and helpful.