Code review comment for lp:~salgado/linaro-image-tools/improve-cmd_runner

Yes, communicate is meant to be used only with pipes and it always
returns (stdout, stderr), but when the subprocess returns non-zero
communicate() will raise (via wait()) before it returns, so I'll have to
think of a way to make stdout/stderr available in that case. Maybe just
including them in the exception is good enough?