target/armv7m: minor fixes of target algo exit point check

target/armv7m: minor fixes of target algo exit point check

Introduce a new ERROR_TARGET_ALGO_EXIT as currently used
ERROR_TARGET_TIMEOUT should be reserved for the timeout only.

Do not load PC directly from CPU HW as the register value
has already been cached.

Change-Id: I0d3630da41fd021676789dc12b52545cc0432ba8
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5329
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>