Fix displaying errors when rustbook tests fail.

Cargo.lock

@@ -3016,6 +3016,7 @@ name = "rustbook"
 version = "0.1.0"
 dependencies = [
  "clap",
+ "env_logger 0.7.1",
  "mdbook",
 ]
 

src/tools/rustbook/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 
 [dependencies]
 clap = "2.25.0"
+env_logger = "0.7.1"
 
 [dependencies.mdbook]
 version = "0.4.3"

src/tools/rustbook/src/main.rs

@@ -9,6 +9,7 @@
 use mdbook::MDBook;
 
 fn main() {
+    env_logger::Builder::from_env(env_logger::Env::default().default_filter_or("warn")).init();
     let d_message = "-d, --dest-dir=[dest-dir]
 'The output directory for your book{n}(Defaults to ./book when omitted)'";
     let dir_message = "[dir]