error-index: Don't generate 404 instead of removing it.

src/tools/error_index_generator/book_config.toml

@@ -7,6 +7,7 @@ src = ""
 git-repository-url = "https://github.com/rust-lang/rust/"
 additional-css = ["error-index.css"]
 additional-js = ["error-index.js"]
+input-404 = ""
 
 [output.html.search]
 enable = true

src/tools/error_index_generator/main.rs

@@ -171,9 +171,6 @@ This page lists all the error codes emitted by the Rust compiler.
 </html>"#,
     )?;
 
-    // No need for a 404 file, it's already handled by the server.
-    fs::remove_file(output_path.join("error_codes/404.html"))?;
-
     Ok(())
 }