"Signature Verification Failed" when flashing stock ROM
I have found sammobile.com a very useful site for finding firmware for samsung devices. I don't think there is an official download site.
Here is the firmware for SGH-M919

I have had the same issue before. Just trying the same process again fixed it for me.
You can try wiping your cache and dalvik cache but I am not sure if that will make a difference. Then retry flashing the ROM.
Also make sure you have downloaded the file correctly. Trying to Install a partially downloaded file will give you a lot of headaches!