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 `$ bin/rake db:create 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.

1 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index 5372f8821f..778175e2e8 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -10,6 +10,13 @@ module ActiveRecord
         @connection.exec_query('create table ex(id serial primary key, number integer, data character varying(255))')
       end
 
+      def test_bad_connection
+        assert_raise ActiveRecord::NoDatabaseError do
+          connection = ActiveRecord::Base.postgresql_connection(database: "should_not_exist-cinco-dog-db", adapter: "postgresql")
+          connection.exec_query('drop table if exists ex')
+        end
+      end
+
       def test_valid_column
         column = @connection.columns('ex').find { |col| col.name == 'id' }
         assert @connection.valid_type?(column.type)