Minor refactor of execute.py
This is a first stab at addressing shrockn's comments in D2286. I have not yet decided what to about the
call to all((isinstance(x, ... yet. I am putting this up now so we can discuss how to best add test
coverage to ensure these refactors are actually being tested.
Test Plan: pytest python_modules/dagster/dagster_tests/core_tests/execution_tests
Reviewers: alangenfeld, schrockn
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D2408