The 502 error indicates a bad firmware flash. You should be able to reinstall the firmware by holding the back, up and middle buttons for ~45 seconds until the watch vibrates. Don't let go of the buttons until the watch vibrates, even if the watch reboots.
Once the watch vibrates it should go back to the pairing screen it displayed the first time you turned it on. Pair it with our phone and Pebble app and it will reinstall the latest firmware.
If you can't get out of the SOS screen, hold the Down button for 20 seconds or so and it should exit. Then do the down, up and select thing.