descriptive error message when AR adapter was not found. Closes #7313

The error message currently looks like:

```
LoadError: could not load 'active_record/connection_adapters/non-existing_adapter'. Make sure to add the necessary adapter gem to your Gemfile:
	sqlite3:	gem 'sqlite3'
	postgres:	gem 'pg'
	mysql:		gem 'mysql2'
```

This is a fix for #7313