doom-debug-p and doom-interactive-p have always been intentionally
redundant, because changing the variables they replaced had other
side-effects, which made writing tests for them difficult. Since our
new (yet unpublished) tests lean heavily toward integration testing more
than unit testing, this becomes an implementation detail.
And doom-init-p's only use was refactor out at some point in the past,
so it's no longer used.
Also done to reduce Doom's footprint, in general.
Produces more helpful (and harder-to-miss) error messages when a hook
emits an error. Also advises run-hook when doom-debug-mode is active, so
errors in hooks (generally, major mode hooks) don't quietly go
unnoticed.
More in line with Emacs' built-in practice of storing a variable's
standard-value in a symbol property of the same name, with the added
benefit of less global state.
