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 <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2365


Tested-by: jenkins
Reviewed-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-by: Jens Bauer <jens@gpio.dk>