Sanity-check SERVO pin for servo probes

Marlin/src/inc/SanityCheck.h

   #if HAS_Z_SERVO_PROBE
     #ifndef NUM_SERVOS
       #error "You must set NUM_SERVOS for a Z servo probe (Z_PROBE_SERVO_NR)."
+    #elif Z_PROBE_SERVO_NR == 0 && !PIN_EXISTS(SERVO0)
+      #error "SERVO0_PIN must be defined for your servo or BLTOUCH probe."
+    #elif Z_PROBE_SERVO_NR == 1 && !PIN_EXISTS(SERVO1)
+      #error "SERVO1_PIN must be defined for your servo or BLTOUCH probe."
+    #elif Z_PROBE_SERVO_NR == 2 && !PIN_EXISTS(SERVO2)
+      #error "SERVO2_PIN must be defined for your servo or BLTOUCH probe."
+    #elif Z_PROBE_SERVO_NR == 3 && !PIN_EXISTS(SERVO3)
+      #error "SERVO3_PIN must be defined for your servo or BLTOUCH probe."
     #elif Z_PROBE_SERVO_NR >= NUM_SERVOS
       #error "Z_PROBE_SERVO_NR must be smaller than NUM_SERVOS."
     #endif