How do I make a Servo Stop if it Hits Resistance?
You could either:


Use a limit-switch to sense that the servo has completed it move. If it doesn't complete it a reasonable time, return it to its original position. Or,
Measure the motor current by putting a low value resistor inline with the ground side of the motor's power supply. With one of the A/D pins, measure the voltage on the motor side of the resistor, which increases with motor current. You'll need to have figured out how much current the motor draws when operating normally. If the motor stalls, your controller could sense that by seeing that current increase. Pick a value (try 150%, for example) of over-current as an indicator that the motor is stalled, and return it to its original position.


The first way is easier to implement but possibly lets the motor stall while you wait for the expected move time to expire. The second way lets you detect stalling and take action immediately.