Add changelog entry for scaffold generator and jbuilder changes

Introduced in 3bfd99defb559af0b017ee920ca714aa1e367fdd.
Related to #8840.

[ci skip]
Bring back "database already exists" messages when running rake tasks

When running tasks such "rake db:setup", instead of showing messages
like "db_development already exists", it was showing a big stack trace
and a message "Couldn't create database for ..." with the configuration
options, a very confusing message with a big trace.

This brings back the functionality present in 3-2, showing the same
message.