using result types in capnp is fraught. while it makes some sense for
interfaces that need neither streaming nor pipelining and can provide
much better error fidelity there it's also fundamentally incompatible
with those that *do* need streaming or pipelining: streams will never
be stopped early unless an exception flies, and pipelines cannot look
through result types. likely the best thing we can do is to encode an
error for transport in the capnp/kj exception description strings. :(
Change-Id: Icb7d16238fa9a7aaf92c00363f7be4076ac02a61
