fix clippy lints and improve error handling

src/main.rs

@@ -348,6 +348,15 @@ fn output_html(
     }
 
     std::fs::create_dir_all("output")?;
+    {
+        let temp_path = "output/calendars.html.tmp";
+        let final_path = "output/calendars.html";
+        let mut f = std::fs::File::create(temp_path)?;
+
+        f.write_all("".as_bytes())?;
+        std::fs::rename(temp_path, final_path)?;
+    }
+
     {
         let temp_path = "output/index.html.tmp";
         let final_path = "output/index.html";