Failure to load task in OOP node hangs MSBuild

[rainersigwald]

This issue exacerbates #764 by changing a failure into a hang.

Since the "failure to load" message gets logged by the main MSBuild.exe, it looks like the failure is getting sent over IPC correctly. But then the parent MSBuild hangs forever in TaskHostTask.Execute, while the task host node is just sitting waiting for work.

Maybe the prior HandlePacket that got a failure didn't correctly set taskFinished?