Add tests for Entity::errors() being able to read data with a path.

Refs #3807
Implement path based error reading.

Much like other parts of CakePHP, errors in entities should be readable
using a dot separated path.

* Implement code.
* Fix tests.

Refs #3807
Fix PHPCS warnings.
Merge pull request #3928 from markstory/3.0-entity-deep-errors

3.0 entity deep errors