failures from certbot.main.main(certbot_args) not handled

Hi!

I've tried the same route you've done in this repository by calling:
```python
certbot.main.main(certbot_args)
```

This caused some problems because certbot might call sys.exit (prematurely exiting the lambda) and the lock file isn't freed.

I fixed it this way:
https://github.com/meeuw/certbot-lambda/blob/master/certbotwrapper/app.py#L33