Avoid incomplete jobs when Git cloning fails

[Martchus]

• Retry Git processing instead of immediately failing the Minion job (and thus also setting the related openQA job to incomplete)
• Improve error messages in git_clone tasks (see separate commit message)
• See https://progress.opensuse.org/issues/163784

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.49%. Comparing base (d9d83ae) to head (245919d). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5803 +/- ## ======================================= Coverage 98.48% 98.49% ======================================= Files 394 394 Lines 38666 38683 +17 ======================================= + Hits 38082 38099 +17 Misses 584 584 ```

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