if a context is set (e.g. through DOCKER_CONTEXT or the CLI config file), but
wasn't found, then a "stub" context is added, including an error message that
the context doesn't exist.
DOCKER_CONTEXT=nosuchcontext docker context ls
NAME DESCRIPTION DOCKER ENDPOINT ERROR
default Current DOCKER_HOST based configuration unix:///var/run/docker.sock
nosuchcontext * context "nosuchcontext": context not found: …
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>