Handle java 'error' event

When running soynode on a system where _java_ was not yet installed, I ran into the following error:

```bash
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:1000:11)
    at Process.ChildProcess._handle.onexit (child_process.js:791:34)
```

This PR adds a handler for the 'error' event, resulting in a readable/catchable error.
according to the npm docs, if we listen on both 'error' and 'exit', we're supposed to have a guard that makes sure you only respond to one of them. see:
http://nodejs.org/api/child_process.html#child_process_event_error