Don't crash if USB volume can't be locked

[ftruzzi]

Hi,

I was trying to flash the unRAID image on Windows and I had a file open in the USB key (actually a previously corrupted unRAID install). The USB creator crashed with an error message: An error occurred when attempted to lock the volume followed by some gibberish characters.

It would be nice to add a "Retry" button so that we don't have to open the creator again and redownload the entire image.

Thanks!