Comment 2 for bug 1287310

We can fix this if we customize the error message of the fields generated by make_form_field() when the field is of type ChoiceField:

http://paste.ubuntu.com/7106647/

With that fix in place, the error looks like:
$ ./bin/maas maas2 node update node-29d7ad70-4671-11e1-93b8-00225f89f211 power_parameters_power_driver="LAN_2_0yoyo"
{"power_parameters": ["Power driver: LAN_2_0yoyo is not a valid power_driver. It should be one of: LAN, LAN_2_0."]}