Config migration guide and some improved error messages
Wrote up a migration guide for the config system.
Some of these errors are definitely a bit obscure and difficult-to-interpret.
Even, dare I say, gnomic.
Took an existing pipeline with a bunch of config schema executable
on 0.6.9 and then ported it over to 0.7.0 element by elementl
Reviewers: max, prha, themissinghlink
Reviewed By: themissinghlink
Differential Revision: https://dagster.phacility.com/D2029