[FIXED JENKINS-9540] Failure of recursive copy on slave side should be reported.

Instead it was being thrown in a Future that the master did not wait for:
the writing side failed with a closed pipe exception, and this meaningless error was thrown up.
Fixing by waiting for the reading side first, so any error there will be thrown up.
(If there was _also_ an error on the writing side, we would miss it, but typically this will just be the closed stream.)