The EWS bots are failing because MSBuild wants the build output directory to exist at launch so that it can immediately start writing logs.
Visual Studio was apparently happy starting up, creating any directories it needed, then starting logging as part of a pre-build step.
I need to revise the Windows build logic to make sure the build target directory exists at the time we trigger MSBuild
Created attachment 247987 [details]
Committed r181098: <http://trac.webkit.org/changeset/181098>