Add human-readable localizable validation failure messages

[vearutop]

Somewhat similar to https://github.com/epoberezkin/ajv-i18n or based on it.

To address the concern raised in opis/json-schema#14.

[skewty]

This would be nice along with #103. Having all errors presented in an easy to process structure and additionally the option (if installed use it, otherwise return English only). Implicitly running a code path to a library like https://github.com/epoberezkin/ajv-i18n is not necessary for some use cases.