There was a bug in the find_feat_part function that would cause it to
fail if the album artist was the second part of the featured string.
For example, if the Artist field was Alice & Bob, and the Album Artist
field was Bob it would return None due to the order.
This fixes that and adds test cases to ensure it doesn't return.
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.
