Auto merge of #4088 - Nemikolh:buildscript-stderr, r=alexcrichton

Write stderr output from build-scripts next to stdout output

Closes #3462.

Please let me know if you want to change the file name for the error output. I originally thought that `stdout` and `stderr` would have been nice but I'm worried that changing `output` to `stdout` would cause breakage. So I choose the more conservative route.