mdBook fails to open the web browser upon `mdbook build --open`, if the first link in the summary is a draft chapter (or no links exist in the summary)

[LikeLakers2]

(Hope you don't mind, but I'm going to use a template similar to Godot Engine's issue template. Makes it easier to read the issue, in my opinion. :D )

mdBook version:
mdbook v0.4.14, installed via cargo install mdbook, compiled via the stable-x86_64-pc-windows-msvc toolchain (currently rustc 1.57.0 (f1edd0429 2021-11-29) as of this issue posting)

Issue description:
mdBook fails to open the web browser upon building the book, when building using mdbook build --open, if one of the following conditions are met:

• The first link in SUMMARY.md is a draft chapter (e.g. [Chapter 1]())
• The SUMMARY.md file is devoid of links.

If either of these conditions are met, then mdBook returns an error upon trying to mdbook build --open:

[INFO] (mdbook): Opening web browser
[ERROR] (mdbook): Error opening web browser: IO error

with no further detail on what this IO error is.

I would expect mdBook to cover these cases, most likely in the following ways:

• If the first link in SUMMARY.md is a draft chapter, then I would expect it to look to the next chapter, and the next, and the next, etc... until it finds a valid link, which it opens the web browser to.
• If mdBook cannot find a valid link, I would expect it to return an error saying as such. For example, "Could not find a valid book page to open to!"

Steps to reproduce:

1. In the SUMMARY.md file of any book (even a freshly-created book), add a new draft chapter above all other chapters, such as [Chapter 1]().
2. Attempt to build with the --open parameter, asking mdBook to open the resulting book afterwards.
3. Observe that mdBook will return a vague "IO error".

Minimal reproduction project:
mdbook-issue-1713.zip

[joshrotenberg]

My guess after looking for a couple minutes: with --open the build command opens the path explicitly to the book's index.html but if the chapter is a draft that file never gets created. Maybe just a better error is the right way to handle this; check that the file exists before calling open and quit with an error from mdbook first.

[LikeLakers2]

@joshrotenberg Please ignore my (now-deleted) comment -- I misinterpreted what was going on, and I also misinterpreted what you said. It seems that what you're saying is right.

That said, while I'm not opposed to your solution (it helps with the issue), I would still prefer if mdBook used the first non-draft-chapter as index.html, instead of erroring out if the first chapter is a draft chapter.