Use table_exists? to check for ActiveRecord models with db tables

It's possible to have models subclassing ActiveRecord::Base that don't
have tables in the database. Using the table_exists? is a better check
than table_name.
Add progress indicator so we can see we're working

If you have a big database, it's nice to see some idea that we're still
working. Also, added the model name before the ID when displaying an
invalid record.