Ading --exit-zero back.  Reason:

nikola/data/themes/default/messages/messages_zh-cn.py:2:41: E901 SyntaxError:
invalid syntax
nikola/plugins/compile_rest/pygments_code_block_directive.py:4:2: E901
SyntaxError: invalid syntax

Both are nonsense.

Signed-off-by: Kwpolska <kwpolska@gmail.com>