Improve get_rate_ctr() error handling

Report back expected interval values.

Change-Id: I05ca7f716342af4f7424b28216ed6c1bf2bd589f
Related: OS#2550
diff --git a/src/ctrl/control_if.c b/src/ctrl/control_if.c
index 590f74d..38e7aef 100644
--- a/src/ctrl/control_if.c
+++ b/src/ctrl/control_if.c
@@ -597,7 +597,7 @@
 		return CTRL_CMD_REPLY;
 	} else {
 		talloc_free(dup);
-		cmd->reply = "Wrong interval.";
+		cmd->reply = "Wrong interval. Expecting 'per_sec', 'per_min', 'per_hour', 'per_day' or 'abs' value.";
 		goto err;
 	}