Fix test which failed Error migration on Windows bots.

Note, this is https://llvm.org/bugs/show_bug.cgi?id=27187.

The problem here was that just converting an error to a bool doesn't
always set the checked bit.  We only set that bit if the Error didn't
actually contain an error.  Otherwise we'd end potentially up silently
dropping it.

Instead just use the consumeError method which is designed to allow us
to drop an error.

llvm-svn: 265311