Ref(SPV-1216) Fix unit tests status codes

[ac4ch]

Pull Request Checklist

• 📖 I created my PR using provided : CODE_STANDARDS
• 📖 I have read the short Code of Conduct: CODE_OF_CONDUCT
• 🏠 I tested my changes locally.
• ✅ I have provided tests for my changes.
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[github-actions]

Manual Tests

ℹ️ Remember to ask team members to perform manual tests and to assign tested label after testing.

[mgosek-4chain]

Here's the solution how we can simplify error checks by using only errors.Is.

#26

[mgosek-4chain]

There is still one thing to improve which I forgot to do in my PR:

1. Add such stub response per each testpkg:

func NewInternalServerSPVError() models.SPVError {
	return models.SPVError{
		Message:    http.StatusText(http.StatusInternalServerError),
		StatusCode: http.StatusInternalServerError,
		Code:       models.UnknownErrorCode,
	}
}

2. Per each unit test add case where SPVError JSON is returned for 500 HTTP status code

For ex.

	"HTTP GET /api/v1/configs/shared response: 500": {
			expectedErr: configstest.NewInternalServerSPVError(),
			responder:   httpmock.NewJsonResponderOrPanic(http.StatusInternalServerError, configstest.NewInternalServerSPVError()),
		},