Custom Validation Errors support

[darekg11]

Make sure that developer using this SDK is able to correctly retrieve custom error message coming from validation / redemption methods. Custom error message is a part of error property.
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