Add an API to provide a way to get errno of juice_send

[mojyack]

The goal of this PR is to allow users to get the failure reason of juice_send.
juice_send only tells whether the transmission failed or not.
However, sometimes knowing the failure reason can help to fix it.

For example, It would be helpful to be able to do like this:

if(juice_send(...) != JUICE_ERR_SUCCESS) {
    if(errno==EAGAIN) ... // retry later
    if(errno==EMSGSIZE) ... // tell user to make packets smaller
    ...
}