Simplify missing_files method

By moving the missing_files method to SystemDescription calling it
and its implementation become simpler
Extract methods to get missing files
Extract method to format missing file errors
Eliminate intermediate map of missing files
Don't return unused value
Deduplicate scope validation code

This requires introduction of a "missing_files_for_scope" method,
which switches a method based on the scope. This clearly should
rather be solved by polymorphism in the scope classes, but this is
a step for later.
Merge branch 'refactor_file_validation'

Pull request #201
Implement method to clean up json error messages

Implement testable method to clean up the weird json error messages
since they are unusable for some scopes like unmanaged-files.
Use cleanup_json_error_message for json validation error handling
Added missing integration test adaption
Merge pull request #205 from SUSE/implement_clean_up_json_error_messages

Implement clean up json error messages
Add file store method to rename
Use tmp directory for extracting unmanaged files

During inspection unmanaged files are extracted to the file store
"unmanaged_files.tmp".
The directory gets renamed as soon as the inspection is finished.

This allows debugging after canceling the inspection and also prevents
overwriting of working data.

If unmanaged_files were extracted before the file store is kept until
the new inspection is finished to keep the system description state
consistent.

Fixes #160
Mention tmp directory during unmanaged files inspection cancelation

Unmanaged files are now extracted to a temporary directory. If the user
cancels the process he gets a message where these partly extracted files
are available.