3.0 avoid fatal error by throwing exception

Throws an exception instead of a fatal error when the datasource classname contain a `\`, for example: 
```php
'default' => [
			'className' => 'Cake\Database\Connectio',
			'driver' => 'Cake\Database\Driver\Mysql',
			'persistent' => false,
			'host' => 'localhost',
			'login' => 'root',
			'password' => 'root',
			'database' => 'cake3a2',
			'prefix' => false,
			'encoding' => 'utf8',
			'timezone' => 'UTC',
			'cacheMetadata' => true,
```

Without this modification, this happen : 
![image](https://cloud.githubusercontent.com/assets/4977112/4042624/12dead84-2d06-11e4-84b2-4bbfa3bcfada.png)

With this fix : 
![image](https://cloud.githubusercontent.com/assets/4977112/4042636/319f49e0-2d06-11e4-8203-65f7b066ffe2.png)