Fix some missing double-quotes around error messages in docker publish plugin

Hi,

Was testing this out and running into the following error when attempting to use the publish to docker feature:

```
Docker Plugin: Missing argument(s)nn
/usr/local/bin/drone: line 40: unexpected EOF while looking for matching `"'
```

After a bit of sleuthing I found that it is trying to echo helpful error messages that are missing a closing double-quote, so users like myself don't get an informative message.

Furthermore, it turns out that the configuration paramswrongly documented. It should be 'docker_server' instead of 'docker_host' in .drone.yml