Implement Error on base receiver

[andig]

Proposed changes

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Types of changes

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Further comments

This allows writing code like

        var oe ocpp.Error
        if errors.As(err, &oe) && oe.Code == ocppj.GenericError {
            ...
        }

which would otherwise need be written using pointer to pointer as in

        oe := new(ocpp.Error)
        if errors.As(err, &oe) && oe.Code == ocppj.GenericError {
            ...
        }