Sadly, I got bitten by this as well. I think the
Please unplug the phone, take the battery out, boot the device into
fastboot mode, and start this script again.
message is super dangerous if it just failed right in the middle of a flash. If the other partitions were there it would probably still be able to reboot, but like this (mix of /e/OS, iodéOS) there is no hope. Not the greatest experience with a new phone 
To end on a more positive note: it is good that Fairphone provides the images at least, which seems to be a pretty recent thing. If there only were a public firehose loader so we could unbrick it ourselves as with FP3 (Fairphone 3 unbricking)…