Modernizes the way body validation works:
Body validators implement the BodyValidator interface.
In case of errors, ValidationExceptions are stored in a List.
Therefore, new methods validate(List<ValidationException>) has been created.
In order to allow IDEs to mark the respective parts of the code, ValidationException carry additional information about the concerned object.

Is trying to retain backwards compatibilty: The old validation methods use their respective Validator and throw the first exception in the list. The old message texts are kept.
PackManager: Only release bodies after writeOut if it is supposed to do
so
Small bugfix in ValidationException (Message not correctly handled) and
clarification in the exception text of UsesValidator
Merge pull request #277 from MarcMil/new-branch2

Modernizes the way body validation works