notify user when opening scenario with an EMANE model that doesn't exist

```
What steps will reproduce the problem?
1. Add an EMANE model X to CORE
2. Save a scenario that uses model X
3. Open scenario on another machine that doesn't have X
4. Open WLAN dialog -- EMANE tab is highlighted, no model selected
5. Pressing Start gives you a cryptic KeyError exception from the daemon

What is the expected output? What do you see instead?

Dialog box warns user that the given model is unavailable.
Daemon logs an exception "EMANE model X not found!"
```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 23 Apr 2013 at 5:52