Device Delete Name check

[ajlacey]

Session.py deleteDevice() does not currently validate that the device name is in the topology. This check should be added to present the front end with an error message if a non-existent device is attempted to be deleted.

This issue is low priority as the input is currently constrained by the front-end to only be able to delete devices that are already created. Additionally, if the device name is invalid, then the function will execute and present a new (but unchanged) topology to the front end. This results in more work, while still leave a valid topology for the user.