There are some readily available error signals. The most suitable one is the feedback potentiometer. The center tap of this gives you a voltage proportional to the current position of the servo. Since you know the commanded position, then you can find out the difference between the two entities and get an error signal. Or you can simply check the current the servo is drawing.
Most simple servos use a pulse length as an error signal.
They don't use a voltage level. You can search the particular servo for this error signal with an osc. NE544 is an old and faithful servo amp. You should be able to find it in many catalogs. Maybe you could compare your circuit to these and see whether HT7001A is actually one of these.