cache formatting not working
If you are familiar with fastboot, you could boot or flash TWRP and format the cache that way.

Download the current TWRP img file, copy it to your fastboot folder, and issue the command

fastboot boot twrp-3.0.2-0-mako.img 

Once it loads and starts at the main menu, select Wipes and then untick everything except Cache and Dalvik cache and execute the wipe and reboot. 

If that fails, it might be time to flash a new factory image.