Added a per-frontend configurable ErrorHandler.
Use the new ErrorHandler in the Controllers.
BlueimpController now uses the correct array key when reading files from FileBag.
Exception tweaks.

ValidationException is now extending Symfonys UploadException and
will be passed through. There is now a new field for adding custom error messages.
Use the blueimp error handler in the tests.
Reimplemented original addToOffset method.
addToOffset is now using arrays instead of variable argument list.
ErrorHandler documentation.
Moved the check for a custom error message entirely to the ValidationException.
Merge pull request #27 from 1up-lab/error-handler

Proper error handling.