stlink: Decode more errors.

stlink: Decode more errors.

For STLINK_SWD_AP_FAULT git://git.ac6.fr/openocd commit 657e3e885b9ee10
returns ERROR_OK with the comment:
Change in error status when reading outside RAM.
This fix allows CDT plugin to visualize memory.

Change-Id: Id8e220961b748ef9467fd84de62b1dc84ace63f8
Signed-off-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-on: http://openocd.zylin.com/3373
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>