sink.text.pretty: print discarded events and packets messages

When getting a discarded events or discarded packets notification in
sink.text.pretty, print a warning on the standard error stream (to
remain compatible with Babeltrace 1 which does that) which looks like
this:

    WARNING: Tracer discarded 83565 events between [20:01:32.062781965]
    and [20:01:32.126649071] in trace "hostname/the-trace" (UUID:
    a560bbc4-7f93-b346-a980-3959745a0f5d) within stream
    "/home/user/my-traces/the-trace/stream-1048576_3_0"
    (stream class ID: 1, stream ID: 3).

The message also indicates when the trace has no UUID, or when the
stream has no (instance) ID. The stream class ID is always available.
When the trace's or stream's name is unavailable, `(unknown)` is
printed.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>