target: fix missing semihosting return path

bug nicely caught by clang.

Change-Id: I7abf0fdd76666fb3eb1c83e3edfd01e0da485ffe
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/359


Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/target/armv4_5.c

@@ -966,6 +966,7 @@ COMMAND_HANDLER(handle_arm_semihosting_command)
 	if (!arm->setup_semihosting)
 	{
 		command_print(CMD_CTX, "semihosting not supported for current target");
+		return ERROR_FAIL;
 	}
 
 	if (CMD_ARGC > 0)