The schema of a JSON-stream message is very pertinent to the api module.
Provide a canonical definition in the api module and refactor the daemon
code to use it. Drop the long-deprecated ErrorMessage field from the API
definition, but have the daemon continue to emit it for compatibility
with docker-py v7.1.0.
Co-authored-by: Cory Snider <csnider@mirantis.com>
Signed-off-by: Austin Vazquez <austin.vazquez@docker.com>