examples: fix missing checks of return values

[gen2thomas]

Solved issues and/or description of the change

One step closer to #1004 by format all examples, so return values are checked now. Additionally the format check is added to CircleCi steps.

Manual test

none

Checklist

• The PR's target branch is 'hybridgroup:dev'
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (e.g. by run make test_race)
• No linter errors exist locally (e.g. by run make fmt_check)
• I have performed a self-review of my own code