Prompted error message is not correct for PortNotFound

When deleting a non-existing port, the error message would be
"Port XXXX could not be found on network None" for some plugins.
"network None" is not correct enough here.

Fixes bug #1203631

Change-Id: Ie5d1ec99a2726ff3fec07f83f83a12d24613ae57
Merge "Prompted error message is not correct for PortNotFound"