Previously, calling public methods on a SolidExecutionResult corresponding to a composite
solid execution threw difficult-to-interpret exceptions, and there was no way to index into child
solid results from the composite result.
Test Plan: Unit
Reviewers: #ft, alangenfeld, schrockn
Reviewed By: #ft, alangenfeld, schrockn
Differential Revision: https://dagster.phacility.com/D1432