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.

Output of both mysql/postgresql:

```shell
./master-app [1.9.3-p327-perf]
$ rake db:setup
masterapp_development already exists
masterapp_test already exists
-- initialize_schema_migrations_table()
...
```

And for sqlite3:

```shell
./master-app [1.9.3-p327-perf]
$ rake db:setup
db/development.sqlite3 already exists
db/test.sqlite3 already exists
-- initialize_schema_migrations_table()
...
```

These changes should hopefully handle the issue @rubys has been seeing
on master related to the rake tasks.

I'm opening as a pull request to get some quick review before applying.
Let me know about any improvements.