iproto: refactor error encoding with mpstream

Refactor iproto_reply_error and iproto_write_error with a new
mpstream-based helper mpstream_iproto_encode_error that encodes
error object for iproto protocol on a given stream object.
Previously each routine implemented an own error encoding, but
with the increasing complexity of encode operation with following
patches we need a uniform way to do it.

The iproto_write_error routine starts using region location
to use region-based mpstream. It is not a problem itself, because
errors reporting is not really performance-critical path.

Needed for #1148