Move invalid step logic down into gRPC, to reduce gRPC calls

Summary: Give up a slightly more typed error in favor of performance, since this is not an expected codepath

Test Plan: BK (see response change in test) - also launch a pipeline run with invalid step keys hacked in and verify that the error experience is reasonable

Reviewers: alangenfeld, yuhan

Reviewed By: yuhan

Differential Revision: https://dagster.phacility.com/D9047