Improve validation reporting

[mikir]

Currently, the validation throws exceptions with message which does not identify uniquely the wrong field. This is because validation is called recursively and there is no way how to get the current "field path".

This can be improved by the implementation of the some path stack or similar which will be passed to each validation method. Or, probably even better, we can invent something like validation report which will be returned instead of the exception. Such approach can be used for reporting several issues in one validation call.