Custom Validation Error support

[darekg11]

Make sure that developer using the SDK is able to pass error property for Validation Rules when creating or updating Validation Rule.
Make sure that error property is correctly mapped when developer is using this SDK to get Validation Rule.
Update README file to include information about custom error messages.

More information can be found in documentation under following links:
https://docs.voucherify.io/reference#section-custom-error-messages-with-validation-rules
https://docs.voucherify.io/reference#the-validation-rule-object
https://docs.voucherify.io/reference#create-validation-rules