server: shutdown command should lead to exit without evaluating the rest

server: shutdown command should lead to exit without evaluating the rest

Currently

openocd -c "echo a1; shutdown; echo a2"

outputs both "a1" and "a2" and only then shuts down. This patch fixes
it by making shutdown command throw an exception, so unless it's
caught the shutdown will behave as expected.

Change-Id: I764268b3a9046ff3e9717d04095ea0673f1d755a
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2511
Tested-by: jenkins
Reviewed-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>