This removes the code that extracts the featured artist from the
original artist field from the ft_in_title function and puts it into its
own.
It also adds a custom ArtistNotFoundException that find_feat_part will
throw if the album artist is not in the artist field.
This allows us to easily test the results of finding a featured artist
in the artist sting, and thus adds tests for the usual test cases that
the split_on_feat gets tested for.