Error flags, error reporting

Error flags, error reporting

- all error flags are in the bit array err_flags, including braking and
  booting (this is unused yet, the idea is to report successful boot and
  pwmled init somehow).
- error status is reported as num_pattern(err, 1) by the status LED
- error status (if any) can be cleared by pressing the button 1
  (which takes precedence over the panic mode)