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.
Deprecate `Rails::Generators::ActiveModel#update_attributes`

Deprecate #update_attributes in favor of `#update`.

ORMs that implement `Generators::ActiveModel#update_attributes` should change
to `#update`. Scaffold controller generators should change calls like:

    @orm_instance.update_attributes(...)

to:

    @orm_instance.update(...)

This goes along with the addition of `ActiveRecord::Base#update`,
introduced in cb2bd4aa619d9329c42aaf6d9f8eacc616ce53f4.
Fix AR tests due to Mysql constant not being defined