feat: Extract out common error into `signstar-common`

Wiktor Kwapisiewiczrequested to merge
wiktor/extract-common-error-variants into main

This moves variants used by both signstar-config and signstar-test into common place.

A question remains whether we want to have a conversion to exit code in there too and if so how to make sure that the error codes are not conflicting between crates 🤔

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