"Verification failed" upon startup after installing software update

My phone installed a software update and upon restarting, I get a 'verification failed' error message. It continues "Unable to restart you device. The integrity verification has failed. You need to reset your device to factory default settings. This will erase all your data." The only option it gives is to 'reset device' - I do not want to do this and lose all my data! I tried powering off and restarting in safe mode, but to no avail. How can the update be so flakey!? At the expense of all my data...? I loaded the recovery screen and wiped the cache. But still no result. Thank you in advance.