Make TableRegistry methods throw exceptions.

When you attempt to reconfigure an existing object in the registry
either though config() or get() we should throw an exception. Prior to
exceptions the updated configuration would not be applied.

Throwing exceptions will help developers find issues where they might
not be getting the configuration data they think they should be getting.

Refs #2336
Fix failing test in DatabaseSession.
Merge pull request #2561 from cakephp/3.0-registry-exception

3.0 - Make TableRegistry methods throw exceptions.