TBR wspr: Change IPC protocol to remove error from out-args.

This is related to:
  https://vanadium-review.googlesource.com/#/c/4751/

The changes in wspr aren't just trivial VDL generation.  Luckily
Benj had the foresight to add TODO comments around most places
where we were hacking things by adding / removing the final error
out-arg.  So I've just found these places and removed the hacks.

AFAICT the javascript code itself doesn't need to change, since
Benj had already set things up assuming that we'd make this
change at some point.  All go tests pass, but I'm just going to
wait for the presubmit to make sure the JS side really passes.

MultiPart: 2/8
Change-Id: Id888b4e86f290bd8931de821c668f4b8829b6c5f
8 files changed
tree: b7624c10ddc0c3fa9fb05b723942b76d8ce0c88b
  1. services/