Importing external network with name duplicated by existing network causes unhandled UI error

When a network from an external provider is imported, and there already exists a network with such a name in the system, the UI breaks due to an unhandled error.

Steps to Reproduce:
1. Create network <network_name> in ovirt.
2. Add external network provider containing a network <network_name>.
3. Open external networks import dialog.
4. Try to import the network with duplicated name <network_name>
5. Unhandled GWT exception occurs.
 
Cause:

ImportNetworksModel.onProviderChosen -> onSuccess(Object model, Object returnValue)  line 153
The return value passed in, is a map external_network:{set of data centers}
For a duplicated network {set of data centers} is empty, which will cause an NPE exception when the Import button is pressed.
This will be in:
ImportNetworksModel.onImport (line 244) -> externalNetwork.getDataCenters().getSelectedItem()