Seems archived. I’m no moderator, but content is as follows …
Well, the boot.img-files are simply not signed. That’s everything.
Sign them and they will boot just well, I already did this for testing my new autobuild-service for rooted boot.imgs (You can just boot one of the images from there if you want).
I don’t think it’s worth the effort for FP to sign their images, since they always get flashed during an update. It’s only for testing. 
Here is how super-bootimg signs the images. You’ll have to generate the keystore before, use the script from AOSP for this (Here is how to use the script).
Edit: I just realized, that @anon36364121 posted the part of the sourcecode from the bootloader, which generates this message. The image will never be checked, the line is commented out (and I think device.is_unlocked is TRUE). I don’t get instantly, why it fails with a non-signed boot.img, still looking into it…
But I double ckecked this now, and it works as I thought before:
First, I tried the original boot-image from the latest 1.8.1-BETA:
fastboot boot orig-boot.img
downloading 'boot.img'...
OKAY [ 0.431s]
booting...
FAILED (remote: bootimage: incomplete or not signed)
finished. total time: 0.432s
Then I signed it* and tried it again:
fastboot boot orig-boot-signed.img
downloading 'boot.img'...
OKAY [ 0.430s]
booting...
OKAY [ 0.011s]
finished. total time: 0.442s
* java -jar keystore_tools/BootSignature.jar /boot orig-boot.img keystore.pk8 keystore.x509.pem orig-boot-signed.img
Other resources …