`mdbook build` flag that doesn't implicitly create files and errors if bad path in SUMMARY.md

[frewsxcv]

For rustc's CI, we set the filesystem to be mostly read-only. If a rustc docs contributor accidentally adds in an invalid path, we get these obscure read-only errors because mdbook is trying to create a file even though we don't want it to. It'd be helpful if there was a mdbook build flag like --nocreate or --strict that doesn't attempt to create files from SUMMARY.md if they don't exist and instead results in a build error.

Does this sound alright? If so, I can implement this.

[steveklabnik]

Seems good to me.

[azerupi]

Yes, sounds good!
I am not sure about the exact flag to use, --strict definitely sounds too generic. --nocreate could be ok, even though it does not clearly show that it only affects the creation of source files. Anyway, this is just a small thing that can be discussed and changed when the PR is ready :)

[frewsxcv]

Opened a PR: https://github.com/azerupi/mdBook/issues/254