Additional help messaging to help the user resolve a conflicted migration

Added additional help messaging for command line scaffold generation. When a migration already exists for the resource, the user is now alerted that they are able to skip the conflicted migration file via the --skip option.

This functionality already exists; I've just updated the help output to remind the user that skipping the migration creation is an option as well.
As we are changing this already, should we use only one string here instead of concat a few. Something like:
```ruby
raise Error, <<-MSG
Another migration is already named #{migration_file_name}:
 #{existing_migration}. Use --force to replace this migration 
 or --skip to ignore conflicted file.
MSG
```