display proper error message

- bug : https://bugs.launchpad.net/linaro-powerdebug/+bug/1014667

Signed-off-by: Sanjay Singh Rawat <sanjay.rawat@linaro.org>
diff --git a/display.c b/display.c
index 94ff2de..e9f4bf6 100644
--- a/display.c
+++ b/display.c
@@ -598,8 +598,10 @@
 {
 	size_t array_size = sizeof(windata) / sizeof(windata[0]);
 
-	if (win < 0 || win >= array_size)
+	if (win < 0 || win >= array_size) {
+		printf("error: invalid window");
 		return -1;
+	}
 
 	windata[win].ops = ops;