faft: Check the new_ec image existed before coping to DUT

Check first to prevent failure of copying to DUT. Give a better comment
for warning.

BUG=chrome-os-partner:12997
TEST=Give an inexistent new_ec image to run firmware_UpdateECBin, see:
firmware_UpdateECBin.dev        [  FAILED  ]
firmware_UpdateECBin.dev        ERROR: Should specify a valid new_ec image for
update, like: run_remote_tests.sh --args "new_ec=/path/to/ec_autest_image.bin".
The ec_autest_image.bin file is included in the firmware_from_source.tar.bz2.

Change-Id: I758de3eb20bfe5e4cfb442a7d67720f74148cbbb
Reviewed-on: https://gerrit.chromium.org/gerrit/33037
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Mike Truty <truty@chromium.org>
1 file changed