Proper error handling.

[epoll-reactor]

At the moment, abilities to handle errors in real-time applications is rather weak.

1. If cURL request fails, the exception is thrown. Moreover, this exception contains only message that something flopped. It is better to return a string or an error code at this layer.

2. If VK API request inside builtin API methods was failed, in all cases except the groups::get_by_id, nothing happens. It makes sense a) to perform logging of occurred error; b) to return some representation of error to user.