Check for deadlinks from the summary during book generation

Previously, any deadlinks from a book's SUMMARY.md wouldn't
cause any errors or warnings or similar but mdbook would simply
create a page with blank content.

This has kept bug rust-lang#47394 hidden. It should have been detected
back in the PR when those wrongly named files got added to the
book.

PR rust-lang#47414 was one component of the solution. This change
is a second line of defense for the unstable book and a first
line of defense for any other book.

We also update mdbook to the most recent version.

src/tools/rustbook/Cargo.toml

@@ -8,5 +8,5 @@ license = "MIT/Apache-2.0"
 clap = "2.25.0"
 
 [dependencies.mdbook]
-version = "0.0.26"
+version = "0.0.28"
 default-features = false

src/tools/rustbook/src/main.rs

@@ -52,12 +52,14 @@ fn main() {
 // Build command implementation
 pub fn build(args: &ArgMatches) -> Result<()> {
     let book_dir = get_book_dir(args);
-    let book = MDBook::new(&book_dir).read_config()?;
+    let mut book = MDBook::new(&book_dir).read_config()?;
 
-    let mut book = match args.value_of("dest-dir") {
-        Some(dest_dir) => book.with_destination(dest_dir),
-        None => book,
-    };
+    // Set this to allow us to catch bugs in advance.
+    book.config.build.create_missing = false;
+
+    if let Some(dest_dir) = args.value_of("dest-dir") {
+        book.config.build.build_dir = PathBuf::from(dest_dir);
+    }
 
     book.build()?;