Export EAGAIN as a memory of Error

[valarauca]

Please discuss your new feature before implementing it if it's nontrivial. Adding a small method is probably trivial, anything larger than that, maybe not. Note that API stability is paramount.

Many of the encoding & decoding functions communicate the state of the decoder/encoders via the posix EAGAIN code (example: 1, 2). This code is even so important, that the documentation calls out how to find it, I can only assume as a way to help consumers of the library.

So why isn't EAGAIN part of the Error enum?

The advantage is obvious it would take

for (stream,packet) in input.packets() {
    if stream.index() == idx {
        match output_decoder.send_packet(&packet) {
            Ok(()) => { },
            // Why does such a common case require so much visual noise?
            Err(Error::Other { code: EAGAIN }) => {
                 // get data out of the decoder
             }
             Err(e) => return Err(e),
        };
    }
}

into

for (stream,packet) in input.packets() {
    if stream.index() == idx {
        match output_decoder.send_packet(&packet) {
            Ok(()) => { },
            Err(Error::EAgain) => {
                 // get data out of the decoder 
             }
             Err(e) => return Err(e),
        };
    }
}

It feels like the most common case that needs special error handling shouldn't require the most visual space.

---

This more closely follows the "flow" of C code examples, e.g.:

rc = send_packet(decoder_ptr, packet_ptr);
if rc == 0 {
    continue;
} else if rc == EAGAIN {
    /* get data out of the decoder */
} else {
    return rc;
}

The very obvious, "I need to read data out of decoder/encoder" return code shouldn't be buried.