Entity::errors() will append errors and will merge the new given errors
with the previous ones already stored in the Entity's $_errors variable.
Also, added a reset functionality to clear all errors.

Entity errors are cleared before the Model::validation() filter function to prevent validation (and consequent write data) with a not empty errors array.
Removed &$self from the filter since it was not used.
ixed tests.

Replacing tests with a more simple, direct approach (from Jails).
Merge pull request #840 from rapzo/feature/entity-appending-errors

Entity::errors() will append errors and will merge the new given errors