PBD-1180 Improve test to check exception output
PBD-1180 Improve test to check exception output

We're relying on the exception message being passed from the Docker
APIError to the DockerControllerException, and on up to the client.

Also it seems we can supply fake or empty values for all parameters to
`start_instance` except for the node name, which is used to look up the
connection object from the `node_connections` dict.

hmrc/captain,hmrc/captain