Try/Except to skip songs that are unavailable

I was getting the following TypeError on the first song that couldn't be
previewed.

Traceback (most recent call last):
File "bandcamp.py", line 188, in <module>
if not Download(track["file"]["mp3-128"], f, "Track " +
str(tracks.index(tra
ck) + 1) + "/" + str(len(tracks))):
TypeError: 'NoneType' object is not subscriptable