Populate or remove Dxl shutdown flag from status dictionary

[hello-binit]

There is a "shutdown" key in the status dictionary that isn't populated or used.

stretch_body/body/stretch_body/dynamixel_hello_XL430.py

Line 62 in e7bb48f

self.status={'timestamp_pc':0,'comm_errors':0,'pos':0,'vel':0,'effort':0,'temp':0,'shutdown':0, 'hardware_error':0,

According to the Dynamixel docs (https://emanual.robotis.com/docs/en/dxl/x/xl430-w250/#shutdown), the shutdown byte can report whether the servo is experiencing:

• an overload error
• an electrical shock error
• a motor encoder error
• an overheating error
• an input voltage error

Confusingly, we also have "hardware_error", "input_voltage_error", "overheating_error", "motor_encoder_error", "electrical_shock_error", "overload_error", "stalled", and "stalled_overload" flags. Furthermore, "stalled" always reports True, even if the servo is clearly not stalled. I think some consolidation and documentation around these error states would be useful.