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
    ...
}

[paullouisageneau]

Thank you for the PR, the idea is very good but I don't really like the API for two reasons:

• The return code is already there to return more specific errors, it feels clumsy to introduce a new function to write a second error code via an argument.
• I don't want to expose platform-specific things in the API. The names EAGAIN or ESIZE are not available on every platform and it would require proper defines, with mapping to WSA errors on Windows for instance.

I think it would be cleaner to simply return new error values like JUICE_ERR_TOO_LARGE. This is retro-compatible since any new negative value should be considered an error by existing user implementations. As a side note, EAGAIN should never be returned.

[mojyack]

Thank you for taking the time.
That makes sense, I'll remake patches.

As a side note, EAGAIN should never be returned.

Could you elaborate on this a little more?
I actually get EAGAIN sometimes when transferring large amounts of data.

[paullouisageneau]

Thank you for taking the time.
That makes sense, I'll remake patches.

Great, thanks!

As a side note, EAGAIN should never be returned.

Could you elaborate on this a little more? I actually get EAGAIN sometimes when transferring large amounts of data.

Oh, my bad, I thought that I changed EAGAIN/EWOULDBLOCK to return success, but I only removed the warning in that case (it happens when the buffer is full). You could introduce a JUICE_ERR_AGAIN for both of them.