Throwing Yast::OSReleaseFileMissingError in OSRelease

- throws an error if os-release file is missing
New changes file entry and new version
Merge pull request #197 from kobliha/os_release_handle_missing_file

Handling missing /etc/os-release file in OSRelease
Update tests for Service
Typos in tests
Add tests for Service + minor fixes
Update changes and version to 3.1.29
Improve method notation in test examples
Merge pull request #198 from vmoravec/update-tests

Add tests for Yast::Service
fixed syntax error in Popup.Feedback()

- 3.1.27
Merge pull request #199 from yast/feedback_fix

fixed syntax error in Popup.Feedback()