Before, that was printed using --debug (and in both hb-shape and hb-view).
Changed it, now hb-shape has a new command-line argument called --show-messages.
When invoked, it also respects other output formatting options.  The messages
are better formatted and printed to te same place that hb-shape output is
directed to.  Previously they were written to stderr.

Fixes https://github.com/behdad/harfbuzz/issues/506