Fix `direnv reload` doing nothing after `direnv deny`

When a user runs `direnv deny` followed by `direnv reload`, the reload
command would silently do nothing because the denied status was only
checked later during the export phase, where it returns early without
error.

Now `direnv reload` explicitly checks if the RC file is denied and
returns a clear error message instructing the user to run `direnv allow`
to approve the content.