voltha system test: extend test suite to check correct indication of download failure

The existing test suite tests/software-upgrades/ONU_Upgrade.robot should be extended to verify also that a download to some non existing file is correctly abborted with the 'download-failed' indication.

So the target is to try to download some non-existing file and to verify the failed state in the command ' voltctl device image list <device-id>'. Should look like this example (e.g. after some succesful upgrade test):

voltctl device image list 8e5f1743-8981-46ab-be03-0732d9d59be0
NAME URL CRC DOWNLOADSTATE IMAGEVERSION LOCALDIR IMAGESTATE FILESIZE
twsh.img http://172.27.170.150:8080/twsh 0 DOWNLOAD_SUCCEEDED /tmp IMAGE_ACTIVE 0
twsh1.img http://172.27.170.150:8080/twsh 0 DOWNLOAD_FAILED /tmp IMAGE_UNKNOWN 0