Status indication

If the mission control team could add a couple of bits to the schematic that would be great. We'll definitely need these but I forgot to mention them when we originally wrote down all the parts of the schematic.
- [x] Some status LEDs (connected to GPIOs on the microcontroller) to indicate whether there's an error or things are OK. In the past I've had e.g. one for "Pyros", one for "Radio", one for "IMU" so that LED can blink if there's an error. We might want to try just having a green "OK" or red "ERROR" LED instead to save parts.
- [ ] A beeper would be a really nice extra. I'm not sure on the best choice here but I'm sure Farnell carries some small beepers (might also be called piezo sounders). Something small and perhaps a single n-channel MOSFET to drive it from the battery voltage would work.