perf report: Show error message when processing sample fails

Currently when perf fails to process samples for some reason, it doesn't
show any message about the failure.  This is very inconvenient for users
especially on TUI as screen is reset after the failure.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1448645559-31167-1-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-report.c

@@ -513,20 +513,26 @@ static int __cmd_report(struct report *rep)
 	if (rep->cpu_list) {
 		ret = perf_session__cpu_bitmap(session, rep->cpu_list,
 					       rep->cpu_bitmap);
-		if (ret)
+		if (ret) {
+			ui__error("failed to set cpu bitmap\n");
 			return ret;
 		}
+	}
 
 	if (rep->show_threads)
 		perf_read_values_init(&rep->show_threads_values);
 
 	ret = report__setup_sample_type(rep);
-	if (ret)
+	if (ret) {
+		/* report__setup_sample_type() already showed error message */
 		return ret;
+	}
 
 	ret = perf_session__process_events(session);
-	if (ret)
+	if (ret) {
+		ui__error("failed to process sample\n");
 		return ret;
+	}
 
 	report__warn_kptr_restrict(rep);