When Stop is returned, the ProgramD3D::GetExecutableTask attempts to pop an
error that has not been set.  Returning Incomplete matches the other error
cases in program binary loading.

BUG=962439

Change-Id: I70895ddc706045620e3df219681982357b7b63e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1620914Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>