Fix #34: Spack doesn't stop when dep fails to build.

- Fixes regression introduced by #30
- Child process needs to exit on error

lib/spack/spack/package.py

@@ -801,6 +801,7 @@ def real_work():
 
                 # Still need to clean up b/c there was an error.
                 cleanup()
+                os._exit(1)
 
             except:
                 # other exceptions just clean up and raise.