Boot failing - Can't restore topic

Hi everyone ! :)

I searched for a case like mine on the forum, but didn't find a solution to my problem, so you're my last hopes !

I have a Nexus 7 (2012) WIFI for a while now and I wanted to return to stock, on Lollipop (I was on Paranoid Android ROM, 4.4.4).
So, l used the Nexus Root Toolkit and ran the "return to stock + unroot" feature.
I downloaded the last google image for my device (5.0.2) and let the thing began.

It erases all partitions like it has to do, but when it needs to write bootloader, it fails and says: "FAILED (command write failed (Unknown error))".
I tried severals time, but always same problem. So I tried to flash stock 4.4.4, but can't write bootloader too: "FAILED (No suck device)" or "FAILED (too many links)" errors appear, something like those.

So, in last hopes, I tried to do this manually and followed this thread: http://forum.xda-developers.com/show....php?t=1907796
I erased correctly all partitions, but when I need to flash bootloader with "fastboot flash bootloader bootloader-tilapia-4.23.img" command, I get "FAILED (command write failed (Unknown error))" too...
I can't flash recovery, get the same error as the bootloader one...

If someone had the same problem or if someone can help me to restore my Nexus 7, it would be very kind ! :)

Thanks to everyone who'll read this ;)