Tell how to Create a Database in Error Message

Currently if you attempt to use a database that does not exist  you get an error:

```
PG::ConnectionBad FATAL:  database "db_error" does not exist
```

The solution is easy, create and migrate your database however new developers may not know these commands by memory. Instead of requiring the developer to search for a solution, tell them how to fix the problem in the error message:

```
ActiveRecord::NoDatabase: FATAL:  database "db_error" does not exist
Run `$ bundle exec rake db:create` and `$ bundle exec rake db:migrate` to create your database
```

Active Record should not know about `rake db:migrate` so this additional information needs to come from the railtie. Potential alternative implementation suggestions are welcome.