process: handle errors

I wanted to try to read stderr and output it in the error message, but it ended up being a bit more trouble than I think it's worth - would have had either include Understream inside of the process stream (circular dependencies, yay) or duplicate some of the functionality that exists in `Understream.split`. I decided that for now just detecting these was good enough, but let me know if you disagree and think it's worth spending more time on.
Where did you find the documentation on the signal argument in the callback? I didn't see it in the node docs I looked at and test for it doesn't seem to be passing?