Bricked your device after manually installing the 4.4.4 file, I did and after days of frustrations finally recovered thanks to some searching and help from researching other forums. The issue is the bootloader that you flash it gives you a mismatch and when the bootloader is corrupt you can not do anything until it is fixed. Trust me I tried adb sideloading roms and it will flash but will not boot. What you are going to have to do is manually install the bootloader from JWR66Y ([GUIDE] Flashing a Factory Image with fastboot / return to stock - xda-developers) as pointed out here ([Q] JWR66Y nakasi factory image bootloader problem - Page 3 - xda-developers) from their you will get a bootloader match. After that you can then manually flash factory image (How to: Flash Nexus 5 Factory Images | Droid Life) but skip the step to install the bootloader from 4.4.4 as it will cause you to go back in a corrupt state. You will find that once the bootloader is correct adb will work like it should and you will be back up and running. This is an issue that has stumped me for 2 days.