Fairly straight forward. It became necessary to wrap `Plugin.Err` with a type
which implements `encoding.MarshalText` in order to have that field rendered
properly in the `docker info -f '{{json}}'` output.
Since I changed the type somewhat I also added a unit test for `formatInfo`.
Signed-off-by: Ian Campbell <ijc@docker.com>
Right now the only client side info we have is whether debug is enabled, but we
expect more in the future.
We also preemptively prepare for the possibility of multiple errors when
gathering both daemon and client info.
Signed-off-by: Ian Campbell <ijc@docker.com>
