Enable localization of error messages

[PetrToman]

Please consider refactoring the code so that localization of error messages is possible. Most strings are in ConstraintError.php, but not all - see JsonDecodingException.php or SchemaStorage.php, for example.

[erayd]

@PetrToman Please feel free to submit a PR for this. This project is extremely short of manpower - so if you're wanting us to implement this for you, it's unlikely to happen any time soon; there are a number of higher priority issues that require attention. If you do the heavy lifting and submit a PR, it'll happen an awful lot faster.