GardenRunner support for premature process termination

If garden has already terminated when we attempt to interrupt it,
do not treat this as an error and proceed to check that garden
really has terminated.

The two cases referred to in the code are discussed here:

https://groups.google.com/forum/#!searchin/golang-nuts/Process.signal$20behaviour/golang-nuts/2Zmut0tcnPE/wIsFGWmWz1MJ