Indicate when a step exited without closing stdout/stderr

As a followup to https://bugs.chromium.org/p/chromium/issues/detail?id=522396, it would be good to know when the recipe engine runs a step which quits, but doesn't close its std\* handles (e.g. it's leaking).

Communicating this to the UI would also be important: being able to see that a step potentially leaked processes/handles would be an important warning to have.
