Failed OTA update Nexus 7 (2012)
Your easiest option is to sideload the update.zip file.

Better option would be to download and flash the system image if you havent done it since 4.4 

Sideload: Download the proper image file and adb drivers for your OS from google and install drivers. Now boot to fastboot by pressing Power + Volume down. Now check if you have connected by typing fastboot devices in terminal/prompt. If your device is listed, continue to flash by running the script file from the downloaded image zip.

This method may work on other non nexus devices but you require proper image files from manufacturer or third party and you may require to go through extra steps before you can use fastboot.

This requires you to have unlocked bootloader. Use fastboot oem unlock to unlock your bootloader.