fix bare except to satisfy newer pycodestyle
See here: https://gitlab.com/fdroid/fdroidserver/commit/5dcb48831feee272a9da4a564c52383d3c952d04#note_42414781
I found this, so I did the change suggested by Ciaran there: https://gitlab.com/fdroid/fdroidserver/issues/262
It's annoying that CI doesn't trigger these, I'm not quite sure which tool is responsible for this though.