stat: getfilecon failure now evokes nonzero exit status

stat: getfilecon failure now evokes nonzero exit status

Add comments and adjust interfaces to allow low-level failure
to propagate out to callers.
* src/stat.c (out_file_context): Return bool, not void,
so we can tell callers about failure.
(print_statfs, print_stat, print_it): Propagate failure to caller.
(do_statfs): Propagate print_it failure to caller.
(do_stat): Likewise.
I nearly forgot to update do_stat to propagate print_it failure,
and it compiled just fine in spite of that.  To prevent possibility
of a repeat, I've marked each function that returns non-void with
ATTRIBUTE_WARN_UNUSED_RESULT.