make sure exit codes get propagated through peru.py and __main__.py

peru.py

@@ -13,4 +13,4 @@ sys.path.insert(0, repo_root)
 
 import peru.main  # noqa: E402
 
-peru.main.main()
+sys.exit(peru.main.main())

peru/__main__.py

@@ -1,3 +1,5 @@
-from . import main
+import sys
 
-main.main()
+from .main import main
+
+sys.exit(main())