Uploading Media does not show an error when connection is interrupted

[pento]

When using the multi-file uploader, it doesn't show an appropriate error message when the connection is interrupted briefly. Instead, it jumps to "Crunching..." and gets stuck there, giving the impression that the upload was successful.

To reproduce:

• Begin an upload - I suggest a file at least a few MB, so you can see the effect more easily. Also, upload to a remote server, so the connection can be easily killed.
• Upload a few %, then disconnect your computer from the network. I found a 15-20 second disconnect worked most consistently. I was testing with an ethernet connection, I assume WiFi will be the same.
• Reconnect, and observe the upload bar. If it continues counting up, try disconnecting again, for a little longer.

If you successfully reproduced the bug, it jumps to "Crunching...", without showing an error message.

I've reproduced this behaviour in 3.4.2 as well as trunk.

I haven't been able to reproduce the same behaviour using the Customizer uploader - I'm not sure if that's due to differences in the JS, or my own timing problems due to the lack of upload progress feedback.

You said this is reproducible in trunk. Was that on media-new.php, or in the Add Media dialog? The latter uses the same (newer) APIs that the customizer uses.

Successfully reproduced in trunk with the Add Media dialogue.