* Close previous log file if one was opened before.
* Return error if opening file fails.

Change-Id: I103025cd86bcac785fe39e13bc7d5f79d78e38e7
Signed-off-by: Girts Folkmanis <opensource@girts.me>
Reviewed-on: http://openocd.zylin.com/3878


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>