Strictly check for SELinux labelling errors

It's essential that files created by vold get the correct SELinux
labels, so make sure to check for errors when setting them.

This will help debug b/269567270.  This is not a fix for b/269567270.

Bug: 269567270
Test: Created user and checked SELinux labels of user's directories
Change-Id: I99e4d530a00f9401532c9cb0990df254b7a12a80