Still cleaning some things up but it's functional.

Translates old config formts (both vim globals and old lua) to the new
names. Also shows the new config in `:healthcheck`

Sometimes deleting a session from the the Telescope picker wasn't
working. It happens when we have extra command files that are filtered
out from the list. Those files break some code in Telescope that finds
the item we want to delete in the picker results. More detail is here:

nvim-telescope/telescope.nvim#3265

We work around the issue by having the Telescope entry maker returning
{} instead of nil. That's not perfect but better than not being able to
delete/

And shows you what the new config should be

We had been relying on `v:errmsg` to catch session errors when restoring
with `silent! source <session>`. The problem with that approach is
`v:errmsg` might be set even when it's an error message that should be
ignored (e.g. by another `silent!` command).

Instead, we now `silent source <session>` (note no !) but if that fails
(which means it's a real error), then we optionally source the session
again, this time with silent! so we load as much of the session as
possible.

Fixes rmagatti#350

Also update `continue_restore_on_error` description