API request: allow `plugin.error()` to take an options parameter

The current API for `plugin.error`/`common.error` is this:

``` js
common.error(message, code, _continue);
```

This API has been good enough for us, but we may want to modify it to take an options parameter for more flexibility:

``` js
common.error(message, options);
```

We would migrate `continue` and `code` into the options (`code` already defaults to 1 and `continue` defaults to false).
