Fix warning output

This has been broken since the progress bar addition.

lib/convert.js

@@ -21,18 +21,23 @@ module.exports = async (cachePath, manifestPath, contentPath, book, concurrentJo
   }
   const pool = workerpool.pool(path.resolve(__dirname, "convert-worker.js"), poolOptions);
 
+  const warnings = [];
   await Promise.all(chapters.map(async chapter => {
     const inputPath = path.resolve(cachePath, chapter.filename);
 
     const destFileName = `${path.basename(chapter.filename, ".html")}.xhtml`;
     const outputPath = path.resolve(contentPath, destFileName);
 
-    await pool.exec("convertChapter", [chapter, book, inputPath, outputPath]);
+    warnings.push(...await pool.exec("convertChapter", [chapter, book, inputPath, outputPath]));
 
     progress.increment();
   }));
 
   pool.terminate();
 
+  for (const warning of warnings) {
+    console.warn(warning);
+  }
+
   console.log("All chapters converted");
 };