Using sys.exit(main()) instead of main()

As we known, Exceptions are raised by the sys.exit() function.
When they are not handled, no stack traceback is printed in the Python interpreter.

In this patch we have known main() got return values(eg:0, 1), but it can't
specifies exit status when terminate the main thread by unusing sys.exit().
So when using sys.exit(main()) instead of main() may be more readable and
reasonable.

Change-Id: I2fbc21b65d62e7d444bd1acf2858114ef7a5995f

tools/install_venv.py

@@ -76,4 +76,4 @@ def main(argv):
     print_help(project, venv, root)
 
 if __name__ == '__main__':
-    main(sys.argv)
+    sys.exit(main(sys.argv))