tools: kwboot: Show 'E' in progress output when error occurs

When kwboot is unable to resend current xmodem packet, show an 'E' in the
progress output instead of a '+'. This allows to distinguish between the
state when kwboot is retrying sending the packet and when retry is not
possible.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>