Catch errors set by send and relay_fw

```reply``` and ```relay_bk``` set a return value of -1:
https://github.com/bumptech/nitro/blob/master/src/Stcp.c#L1400
https://github.com/bumptech/nitro/blob/master/src/Stcp.c#L1463
... making the check for (return_value < 0) valid.

In contrast, ```send``` and ```relay_fw``` set a return value equal to ```nitro_queue_push``` :
https://github.com/bumptech/nitro/blob/master/src/Stcp.c#L1353
https://github.com/bumptech/nitro/blob/master/src/Stcp.c#L1434
... and ```nitro_queue_push``` only return ```NITRO_ERR_EAGAIN``` or 0:
https://github.com/bumptech/nitro/blob/master/src/queue.c#L112

So as the bindings stand, I don't see it possible for us to catch errors in send and relay fw.