catkin_build: print log if stderr is not empty (linux side only), see #1

As discussed in #1, opening a pull request to make discussion easier. Not ready for merge! This is merely a testbed to see if stdout/stderr interleaving poses an issue.

If something got printed on stderr by a build command, the user will want
to know about it. So gather stderr separately from stdout and check if
it is non-empty when the command finishes.