Topic guide: error handling

**[Original report](https://bitbucket.org/flyingcircus/nagiosplugin/issue/1) by Christian Kauhaus (Bitbucket: [ckauhaus](https://bitbucket.org/ckauhaus), GitHub: [ckauhaus](https://github.com/ckauhaus)).**

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*     what CheckError is good for
*     unhandled exceptions
*     avoid second-order errors in Summary (e.g., by accessing non-existent results)
