Added test, but I cannot get it sys.exit to work correctly
Added proper exit-code-checking test for restart issue.

When the process is restarted (and results in an error),
we can now properly check the exit code that comes out of
the 'root' buildout process.
Fixes #73: propagate exit code from restart.

A restarted project's exit code is now properly propagated. This makes
buildout more reliable in automatically-run situations.
Simply re-raising SystemExit works, too
Simplification suggested by @mgedmin
Merge pull request #89 from reinout/sys-exit-when-restarting-fix

Fix for #73: proper sys.exit code when restarting buildout