playground: don’t log the output of a program when it cannot be cached
playground: don’t log the output of a program when it cannot be cached

This was causing StackDriver to interpret play program output as
application output, polluting our logs.

Change-Id: I5deee0012131d4933c5cfa854c71833999402256
Reviewed-on: https://go-review.googlesource.com/90635
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/playground,golang/playground,golang/playground