Check error handling

[jensdrenhaus]

Prefer defining own error types over returning ones from 3rd parties.
E.g. not return errors from github.com/go-playground/validator/v10 but wrap them in own types.

Motivation: Returned errors, including rapped errors, become part of the public API. This makes it tricky to replace the respective 3rd party pkg later on without potentially braking user code