fscan exception handling

id26/controllers/fscan_controller.py

@@ -395,6 +395,9 @@ class ID26FScanController:
             #mot_group.move(*backlash_movement)
 
         nbsteps = round((energy_end - energy_start) / (energy_step))
+
+        assert nbsteps>0, f"energy step too large ({energy_step}) for that energy scan range ({energy_end - energy_start})"
+        
         nbpoints = int(nbsteps) + 1
 
         if self.fscan_par["id_linked"] == 1: