create_join_table uses same logic as HABTM reflections

Before this change, create_join_table would not remove the common prefix
in the join table name, unlike ActiveRecord::Reflections. A HABTM
between Music::Artist and Music::Record would use a table
music_artists_records, while create_join table would create
music_artists_music_records.
Fixed typo in comment
UrlGenerationError are not catched as 404 anymore