Warn on @pipeline function returning value

Summary:
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:

1. Warn on @pipeline returning something and
2. 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
warning.

Test Plan: BK

Reviewers: alangenfeld, max, dgibson, sandyryza

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D4884