Add field name to unexported field name error

[wazazaby]

Hi!

Improvement

• On ErrUnexportedField, add the field name causing the problem to the error message

Other

• Bump github.com/stretchr/testify/require to be able to use require.ErrorIs in tests
• Use named helpers to check error presence in tests (require.NoError & require.Error)

[vrischmann]

The CI workflow is so old it doesn't even start.

Tested locally, LGTM, thanks