[core] introduce abandoned state to ActiveExecution
Change the behavior for steps that are downstream from failure to no longer emitting a structured STEP_SKIPPED event - keeping them in the original "not executed" state instead.
Test Plan: update existing tests
Reviewers: dgibson, schrockn, catherinewu
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D4655