Validate existence of meta data for extracted files

Implement method that returns a list of additional files in relation to
a given list.
Use the additional_files method to create a list of files that have no
meta data.
Refactor the error output method to get the scope title only once.
Add hint about file meta data validation to man page

Add information that we validate the existence of meta data for all
files that exist in the file store.
Merge pull request #214 from SUSE/validate_existence_of_meta_data_for_extracted_files3

Validate existence of meta data for extracted files