ThrowIfHttpError method to be called in all request-posting helpers (previously, they would usually ignore errors).
Added missing REST error checks to Delete, Post and Put methods.
The former behavior would conceal the errors by silently ignoring failures, which looked innocent for methods like DELETE which expect no data in return.
Also, for raw methods (like PostRawAgentRequirement) where user has to guess the format, it's crucial to know the outcome.