Show error message box when failed to save file

For #390

src/main.js

@@ -327,6 +327,10 @@ allowProtocolDialog.init(mainWindow);
 ipcMain.on('download-url', (event, URL) => {
   downloadURL(mainWindow, URL, (err) => {
     if (err) {
+      dialog.showMessageBox(mainWindow, {
+        type: 'error',
+        message: err.toString()
+      });
       console.log(err);
     }
   });

src/main/downloadURL.js

@@ -35,7 +35,7 @@ function getAttachmentName(headers) {
 
 function saveResponseBody(response, filename, callback) {
   const output = fs.createWriteStream(filename);
-  output.on('close', callback).on('error', callback);
+  output.on('close', callback);
   switch (response.headers['content-encoding']) {
   case 'gzip':
     response.pipe(zlib.createGunzip()).pipe(output).on('error', callback);