When an unknown error occurs set the HTTP status code to 500
Merge pull request #299 from rancherio/send-500-error

When an unknown error occurs set the HTTP status code to 500
Simplify how the Docker containers are handled.

This is done by introducting the concept of "namespaces" of container
names. The aliases of a container are under this namespace. Namespace
names are of the form:

//<namespace>/<alias>

This allows us to (within cAdvisor) query all docker containers as
//docker regardless of whether this is a systemd or a non-systemd system.

This does break our ability to handle Docker aliases with the /container
endpoint. I think this is acceptable as our support there was not
consistent between system types.