Server-side rollback can take advantage of the rollback-specific update
parameters, instead of being treated as a normal update that happens to
go back to a previous version of the spec.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
Modify the service update and create APIs to return optional warning
messages as part of the response. Populate these messages with an
informative reason when digest resolution fails.
This is a small API change, but significantly improves the UX. The user
can now get immediate feedback when they've specified a nonexistent
image or unreachable registry.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
