Include details of validation exceptions in response in ValidationExceptionMapper

[slavb18]

This PR contains of

1. addMessageToResponse flag to include validation errors in response
2. messageTemplate to set custom message format

[sberyozkin]

Unfortunately we can not introduce Spring dependencies into this code. The simplest option to customize is to register a custom ValidationExceptionMapper

[slavb18]

ok, thanks

[slavb18]

and how about addMessageToResponse?

[sberyozkin]

I've applied the patch with the minor modifications, thanks. I've only made StringBuilder instantiation optional, and kept the original message format. I'm open though to add 'Value of' - how does the message would read with it ? I hesitated because a constraint violation has a method like .getInvalidValue()