Override Error Message in Create Subnet Workflow

The Admin->Networks->Create Subnet and Project->Networks->Create Subnet
both inherited their error message from the Project->Create Network
workflow, this patch moves the error message into a instance variable
that can be overridden by sub-classes, and overrides it in the Create
Subnet workflow.

Change-Id: I87e9b89b68560291e389a24be86f788167c5d149
Closes-Bug: 1375783
Merge "Override Error Message in Create Subnet Workflow"