stlink: handle error GET_IDCODE

stlink: handle error GET_IDCODE

When the connection with target is lost on the physical JTAG/SWD
link (either wire disconnected or DAP/TAP in reset), STLINK fail
to connect and returns error STLINK_JTAG_GET_IDCODE_ERROR.

Add it in the list of known errors.

Change-Id: Iabd6e6892111726f614345497d9286554bfe4ac0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4982
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins