log error message if specified theme directory does not exist

Author: clement2026

src/config.rs

@@ -580,7 +580,13 @@ impl HtmlConfig {
     /// directory is not present it will append the default directory of "theme"
     pub fn theme_dir(&self, root: &Path) -> PathBuf {
         match self.theme {
-            Some(ref d) => root.join(d),
+            Some(ref d) => {
+                let dir = root.join(d);
+                if !dir.is_dir() {
+                    error!("theme dir {:?} does not exist", d);
+                }
+                dir
+            }
             None => root.join("theme"),
         }
     }

src/renderer/html_handlebars/hbs_renderer.rs

@@ -480,10 +480,7 @@ impl Renderer for HtmlHandlebars {
         trace!("render");
         let mut handlebars = Handlebars::new();
 
-        let theme_dir = match html_config.theme {
-            Some(ref theme) => ctx.root.join(theme),
-            None => ctx.root.join("theme"),
-        };
+        let theme_dir = html_config.theme_dir(&ctx.root);
 
         if html_config.theme.is_none()
             && maybe_wrong_theme_dir(&src_dir.join("theme")).unwrap_or(false)