Throw an error when Chromium download is failed (#619)

This patch teaches ChromiumDownloader to throw an error if
downloading failed.

utils/ChromiumDownloader.js

@@ -98,9 +98,11 @@ module.exports = {
     return downloadFile(url, zipPath, progressCallback)
         .then(() => extractZip(zipPath, folderPath))
         .catch(err => err)
-        .then(() => {
+        .then(err => {
           if (fs.existsSync(zipPath))
             fs.unlinkSync(zipPath);
+          if (err)
+            throw err;
         });
   },
 
@@ -234,4 +236,4 @@ function requestOptions(url, method = 'GET') {
   }
 
   return result;
-}
+}