This is a precondition for the @pipeline @composite_solid
merging. Previously we were ignoring return values from @pipeline. However
as embeddable artifacts with outputs those return values will have meaning
and it will break existing code. Plan will be:
- Warn on @pipeline returning something and
- Special case behavior on the @pipeline decorator until we decide to break
people who have returned this.
This also converts pipelines in dagster and dagster-graphql to respect this