if one upload fails just show the error page for that one

... and continue on with the other uploads. before the whole page would
die...
fix so audio_file input for other-file always gets typeahead
set track.scanned the very last thing we do

because the way we update the track sqlalchemy runs 3 different queries
so before an artist would have been set scanned could have already been
set to true which led to race conditions in upload (at least).