EMANE failure needs to be indicated to the user via CEL or Exception Message

```
What steps will reproduce the problem?
1. create scenario with some MDR nodes linked to a WLAN
2. change WLAN to use EMANE 802.11 model
3. configure 802.11 MAC unicast rate of 54M
4. press start

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

EMANE failed to load but everything appears OK from the GUI. CEL light should
be lit with the exception output shown in the CEL dialog.

Looking at the emane.log file shows the problem:

cat /tmp/pycore.41676/emane.log
13:36:56.147485 ABORT StartException: IEEE80211abgMacLayer: Invalid
Configuration :unicast data rate index must be [0-4] for modulation type 80211B

```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 18 Mar 2013 at 8:38