Replace JSON parser error handling unit tests

Use real json files and the concrete output to test the error handling.
Add validation error unit tests for config_files

Add test json files for config_file scope error handling. They are
identicial with the ones from the changed_managed_files.
Add validation unit test for unmanaged_file error handling
Optimize the schema error output for config_files

Filter duplicate messages and remove entries which are most likely not
related, if there is more than one.
Replace schema error message positions

The position information in schema error messages is confusing and way
to long. Replace them by a short understandable term.
Optimize error cleanup comment
Merge pull request #169 from SUSE/write_unit_tests_for_json_parsing

Write unit tests for json parsing