Exit if a preprocessor command is not found

[Bktero]

A small change discussed in issue #2493

When a command is not found, log an error and exit the application instead of simply logging a warning. Indeed, the generator of the book will probably be wrong anyway, so exiting seems legit.

[ehuss]

I don't think this is something we can do right now, since I believe there are some setups where it is expected to be allowed. Perhaps we could introduce an optional field like there is for renderers (like #1122)? Although that would need to default to false for now.

[szabgab]

I think we could add an optional field as @ehuss suggested that would turn these warnings into errors. By default it would be false, but when a new book is created with init then it would be added to the book.toml file and it would be true.
Suggested name of the field: error_on_missing_preprocessor

[Bktero]

I have added a commit that tries to implement this flag. Is this something like this you had in mind?

There are still 2 points that I need to figure out:;

1. The flag isn't written to book.toml by the init command.
2. The flag is considered to be true is missing from the file, because this is the default config. I think I can simply change it to false, and change init to force it to true.

[Bktero]

I think I figured it out. I added another commit. Here is how I tested it.

First I created book:

$ ./target/debug/mdbook init my-new-book    

Do you want a .gitignore to be created? (y/n)
n
What title would you like to give the book? 
test
2025-03-22 14:56:11 [INFO] (mdbook::book::init): Creating a new book with stub content

All done, no errors...

The following book.toml was generated:

[book]
authors = ["Pierre Gradot"]
language = "en"
multilingual = false
src = "src"
title = "test"

[build]
build-dir = "book"
create-missing = true
error-on-missing-preprocessor = true
extra-watch-dirs = []
use-default-preprocessors = true

I add this at the end of te file (as in the linked issue):

[preprocessor.mermaid]
command = "not-a-real-binary"

I built the book and got an error:

$ ./target/debug/mdbook build my-new-book
2025-03-22 14:58:04 [INFO] (mdbook::book): Book building has started
2025-03-22 14:58:04 [ERROR] (mdbook::preprocess::cmd): The command "not-a-real-binary" wasn't found, is the "mermaid" preprocessor installed?

I changed the flag to false and I built again:

$ ./target/debug/mdbook build my-new-book
2025-03-22 14:58:19 [INFO] (mdbook::book): Book building has started
2025-03-22 14:58:19 [WARN] (mdbook::preprocess::cmd): The command "not-a-real-binary" wasn't found, is the "mermaid" preprocessor installed?
2025-03-22 14:58:19 [INFO] (mdbook::book): Running the html backend

[Bktero]

I have synchronized my fork with the rust-lang's repository and resolved the conflicts.

Any opinions on this PR?