Include process STDOUT in CommandFailed exception message (#4)
Include process STDOUT in CommandFailed exception message (#4)

When a native git command fails, having the `STDOUT` in addition to the
`STDERR` might help diagnose the problem, especially if the `STDERR` is
empty.

Specifically, we've seen cases where `git fetch` invocations exit
non-zero with no `STDERR` reported within the exception message.
brynary/grit,brynary/grit