Corrected error message when no zip-exportable content is selected.
Distinguish error messages from selected items export and export on context.
Added error message when the content is too big to zip.
This happens when the ZIP64 isn't available and the content is bigger than 4GB.
Removed unnecessary super calls.
Merge pull request #22 from 4teamwork/lk_correct_error_message

Better error message on empty containers.