Replaced empty error message when generating owls file from diagram containing unassociated inputs/outputs

Before:

![image](https://f.cloud.github.com/assets/2272207/1111238/e3ac7386-19a4-11e3-979c-f0a3cd9caeb0.png)

After:

![image](https://f.cloud.github.com/assets/2272207/1111240/eda1623e-19a4-11e3-9a27-276a1bc196c0.png)