Refactor list_file_store_content

Rename list_file_store_content to list_description_content and move
from lib/system_description_store.rb to lib/system_description.rb due
to the refactor in pull-request #201.
Add additional_files method

This method returns a list of files that have no meta informations or
invalid meta informations e.g. files in deleted state.
Implement test cases for the validation of existence meta data
Implement the check for files that have no meta data

Use the additional_files method to get a list of files that have no
meta data.
Refactor the error output method to get the scope title only once.