Caterpillar s50 stuck at fastbood mode - bootimage is not signed
Had the same problem flashing and trying to boot custom made boot.img.
I believe you ARE able to enter recovery (if you have not corrupted both boot recovery images, what is unlikely).

To enter recovery on Cat s50 you need to:


Power off the phone (in your case just hold the power button for several seconds).
Press and hold "volume down" + "camera" (the one between volume up/down) buttons. This is tricky by itself and not so easy from first attempt :)
Holding buttons from previous item you should insert the usb cable into powered usb port (controller or host os seems to be irrelevant, just need to have 5V).
Screen will show logo, but will not blink with our "bootimage is not signed" error. You may release the buttons and wait several seconds - it will show recovery menu.


Now. To restore /dev/block/mmcblk0p24 ("boot") nothing helps but factory update image. I had backed up this partition image before re-flashing - it did not helped - same error with one from back-up.
So:


You should download factory image, e.g. from this untrusted source :) https://mega.nz/#F!scdiVLgJ!K7BNoVUTNqaAELWQjsOUnA
There are 2 ways - via adb or sd card - both work fine. Lets describe adb. Go to recovery mode with phone connected to pc (linux in my case). You should select "update via adb" in recovery menu - it will wait till you provide image via adb.
From your pc you enter adb sideload /yourdownloadfolder/update-LTE_S0201121.0_S50_0.023.00-user.zip
Just wait for phone to validate it and restore images (it will say).
Reboot and all should be OK.


Google how to install adb if you don't have it already.
To update from sd card just copy that zip file into sd card root and select it with according recovery menu item. Someone recommend placing CID file (also available on link from "1"), but I did not - all OK.

Hope it will help, and if so - need to investigate further how to sign that damn bootimage. I want it to provide network file system support for this phone - all modules are ready and work just want it to be smooth from boot via custom bootimage.