Remove duplicate error message "Couldn't find..."

This commit removes the duplication of the error message:

> Couldn't find #{@klass.name} with [#{arel.where_sql}]

introduced in #15791 by adding a private method `find_nth!` that
deals with all the method like `first!` and `second!`.
Merge pull request #17251 from claudiob/remove-duplicate-exception

Remove duplicate error message "Couldn't find..."