added code to let learned_termination be created using already existing ensemble as an error population, just copy / pasting the code from hpes_termination template
Merge branch 'master' of https://github.com/ctn-waterloo/nengo
fixed exception catch for Exception thrown by net.get instead of StructuralException
changed hpes_termination to work with the subnet.node notation used in the python API so that subnetwork nodes can be connected to for error signals