Create `RestExceptions.FluentValidation`

[Stratis-Dermanoutsos]

For starters, handle ValidationException.

Quite easy to get the validator's message.

As for determining which type of RestException to throw, use ErrorCode, if possible.

RuleFor(x => x.Property)
    .WithErrorCode("400") // throws BadRequestRestException
    .WithErrorCode(nameof(BadRequestRestException)) // throws BadRequestRestException
    .WithErrorCode("BadRequest") // throws BadRequestRestException