lei: ->child_error less error-prone

lei: ->child_error less error-prone

I was calling "child_error(1, ...)" in a few places where I meant
to be calling "child_error(1 << 8, ...)" and inadvertantly
triggering SIGHUP in script/lei.  Since giving a zero exit code
to child_error makes no sense, just allow falsy values to
default to 1 << 8.