The current one is kinda... fugly.

I've made a new one, which looks like this:

You can download them if you wish at https://github.com/hammerhead-n5/bootgraphics/tree/master/ubuntu-dark

I've made another boot graphic, which looks like this:
. You can download it here.

After you do the switch install, try

fastboot devices

again.
You should see something like

List of devices attached
01a819eb0c59b252	device

If so, then open ubports-installer and follow the instructions. Select "Select device manually", then "Nexus 5". From here on, install as normal.