Add Error Checking to MaestroThrusterController class

[ryan-summers]

The Pololu Maestro motor controller outputs certain errors when serial communication breaks down or motors appear unresponsive. These errors are persistent in microcontroller memory until read, and they result in a red light being displayed on the microcontroller.

Implement a method that periodically reads and records thruster controller error codes.

class MaestroThrusterController
{
    //....
    uint16_t readErrorCode()
    {
        /*
         * Error code is read here.
         */
        return read_error;
    }
};

[irwineffect]

I took a look at the possible error codes: https://www.pololu.com/docs/0J40/4.b

Most of them are basically "did a serial error occur?", which if serial errors are occurring we likely won't be able to talk to the maestro to get the error code anyway. The maestro doesn't have an error for non-responsive motor. Was there some other error you had in mind? If not, I don't see a strong need for this.