helper/command: return proper JIM error code

helper/command: return proper JIM error code

The function command_unknown() is expected to return a value
recognized as JIM error code, as it is correctly done in the
other cases it returns.

Fix the only case in which command_unknown() does not return
a JIM error code, by s/ERROR_FAIL/JIM_ERR/

Change-Id: Ib98b75755ae36870bd68c17f8839ddbfa06c6312
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4973
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>