Make a failed extension load throw an error during pre-compilation

[topolarity]

Without this change the tests added in #56666 pass (even w/o that PR applied), because the failure to load ExtAB is just ignored.

I think we should tighten that up (w/o a backport, so that we don't break folks depending on this). The stab I've taken here might be the wrong approach since I'm not sure how it interacts with any locks, etc., but I wanted to get the ball rolling.

[IanButterworth]

PrecompileableError is special for indicating a package doesn't want to be precompiled.
This should be a regular throw.

However I'm not sure how clear the error would be with this. Worth checking.

Suggested change

	errs = current_exceptions()
	@error "Error during loading of extension $(extid.id.name) of $(extid.parentid.name), \
	use `Base.retry_load_extensions()` to retry." exception=errs
	if JLOptions().incremental != 0
	# during incremental precompilation, this should be fail-fast
	throw(PrecompilableError())
	end
	if JLOptions().incremental != 0
	# during incremental precompilation, this should be fail-fast
	rethrow()
	else
	errs = current_exceptions()
	@error "Error during loading of extension $(extid.id.name) of $(extid.parentid.name), \
	use `Base.retry_load_extensions()` to retry." exception=errs
	end