Fix error replies, improve header marshalling, marshal all header fields

[MaxVerevkin]

Fixes #117

[MaxVerevkin]

I think marshal_header_field may be improved or replaced with something different, because 1) as it is, it requires a few technically unnecessary clones - for destination, interface, member, error_name etc. 2) it is always called with a known variant of HeaderField, so the match is sort of useless (maybe it is optimized out if marshal_header_field is inlined?). A marshal_header_somthing for each field wouldn't be too bad, right?

[KillingSpark]

Thanks for all the code review! I'll take a look soon, right now life is pretty busy. Shouldn't be too long though