I'm having a very similar problem. I was able to install TWRP just fine. However, during the installation of Cyanogenmod 10.2, it gave me these error messages: "assert failed: run_program("/sys/bin/loki.sh," "E:Error executing updater binary in zip '/sdcar", and "Error flashing zip '/sdcard/cm-10.2.0-d800.zip.'" Now, whenever I turn my phone on, it displays the LG logo with "Security Error" written underneath it. I tried doing a factory reset in TWRP, but that didn't fix the problem.
# This leverages the loki_patch utility created by djrbliss which allows us
# to bypass the bootloader checks on jfltevzw and jflteatt
# See here for more information on loki: https://github.com/djrbliss/loki
mkdir -p $C
dd if=/dev/block/platform/msm_sdcc.1/by-name/aboot of=$C/aboot.img
/system/bin/loki_patch boot $C/aboot.img /tmp/boot.img $C/boot.lok || exit 1
/system/bin/loki_flash boot $C/boot.lok || exit 1
rm -rf $C
I ended up using this guide
to revert my G2 to its original state. While I'm not sure why Cyanogenmod failed to install, I suspect it may be because the version I used was not designed for the most recent OTA update (10o).