Find a way to report automatic halts

[saulrh]

When pciod detects that the arms go over current, it halts the entire group. These halts should be reported to programs via somatic_motor_update so that they can be properly handled.

[saulrh]

Found a workaround: with reasonably high accuracy and precision, a near-zero current message indicates that the brakes are on and the motors are disengaged.

[cerdogan]

Instead of this workaround, we should implement the status messages that are already defined in somatic to express any errors.