server/server: Remove all exit() calls

With this patch OpenOCD shuts down properly when errors occur in the
server instead of just calling exit().

Change-Id: I2ae1a6153dafc88667951cab9152941cb487be85
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3223


Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins