DBS_Exception's should include entity name, and optionally Identifier

[e.mortoray]

To aid in debugging, particularly in PHP where the stack track isn't always helpful without looking into the generated code, the DBS_Exception's should also include the entity name of the object related to the exception. Where possible it would also be great to include the identifier of the object.

In some cases multiple entities may be relevant, such as saving the id of an entity field during saving. I guess just the most immediate entity should be used (otherwise it might get hard to do).