Should define a Error type rather than generate error type for every struct

[yjhmelody]

The generated error type is not very useful.

[greyblake]

The idea of a specific error for every particular struct is that:

• Looking at the definition of error type one can easily say which validation rules are there
• It enables exhaustive error handling

In the further versions I am planning to introduce something like GenericValidationError, so every specific validation error will be convertable into GenericValidationError