Suppress most load_property_from_file logs

Some partitions may not have build.prop files (and it's totally fine).
Rather than logging such failures as WARNING, load_property_from_file
now returns a Result<void> object so callers can manually log for
themselves.

Bug: 228389036
Test: boot and logcat
Change-Id: Ia4ad7eef34d572182a3f6e86547664ae31dfd29f