Join model scaffolder throws an error if you don't supply class names

[jagthedrummer]

For simple cases we should be able to guess a class name, and if we can't we should output a friendlier message explaining what the user should do.

This is what happens currently:

$ rails generate super_scaffold:join_model Assignment membership_id task_id
Generating model with 'bin/rails generate model Assignment membership:references task:references'
Updating './app/models/assignment.rb'.
Updating './app/models/assignment.rb'.
/Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/attribute.rb:180:in `partial_name': Invalid field type: . (RuntimeError)
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/transformer.rb:656:in `block in add_attributes_to_various_views'
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/transformer.rb:632:in `each'
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/transformer.rb:632:in `each_with_index'
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/transformer.rb:632:in `add_attributes_to_various_views'
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/bullet_train/super_scaffolding/scaffolders/join_model_scaffolder.rb:81:in `run'
        from /Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/gems/3.3.0/gems/bullet_train-super_scaffolding-1.7.11/lib/scaffolding/script.rb:268:in `<main>'
        from <internal:/Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:37:in `require'
        from <internal:/Users/jgreen/.asdf/installs/ruby/3.3.2/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:37:in `require'