Don't ever prompt to create a headline when transcluding

[josephmturner]

The default setting of org-link-search-must-match-exact-headline causes Emacs to prompt the user to create a new heading when a link target is not found.

This commit temporarily overrides the default setting to signal an error when no headline exactly matches the link target. Since errors are demoted, this skips the failed transclusion.

However, if the user has set org-link-search-must-match-exact-headline to perform a fuzzy search for link targets, we don't override the option.

[josephmturner]

Thanks!

[nobiot]

@josephmturner Thank you!