drivers/stlink: clarify "init mode failed" message

The message as it was didn't let the user know that something was wrong
with the target or wiring.

Change-Id: Ib609c2d31959e77413e61c348d0e31d7269d5c58
Signed-off-by: Paul Fertser <[email protected]>
Reviewed-on: http://openocd.zylin.com/2365
Tested-by: jenkins
Reviewed-by: Uwe Bonnes <[email protected]>
Reviewed-by: Jens Bauer <[email protected]>

Author: paulfertser

src/jtag/drivers/stlink_usb.c

@@ -1831,7 +1831,7 @@ static int stlink_usb_open(struct hl_interface_param_s *param, void **fd)
 	err = stlink_usb_init_mode(h, param->connect_under_reset);
 
 	if (err != ERROR_OK) {
-		LOG_ERROR("init mode failed");
+		LOG_ERROR("init mode failed (unable to connect to the target)");
 		goto error_open;
 	}