Report the system error on why chmod(2) failed in XDG_RUNTIME_DIR

This is a very rare occurrence: if the user is the owner of the
directory, the user can chmod(2), and we already checked that the user
is the owner. However, chmod(2) can still fail on read-only fs and on
hardened systems.

Task-number: QTBUG-41735
Change-Id: I8f8bac763bf5a6e575ed59dac55bd265e5b66271
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>