unset _resource_pickup on error

Author: rickwierenga

pylabrobot/liquid_handling/liquid_handler.py

@@ -1800,10 +1800,14 @@ async def pick_up_resource(
     for extra in extras:
       del backend_kwargs[extra]
 
-    await self.backend.pick_up_resource(
-      pickup=self._resource_pickup,
-      **backend_kwargs,
-    )
+    try:
+      await self.backend.pick_up_resource(
+        pickup=self._resource_pickup,
+        **backend_kwargs,
+      )
+    except Exception as e:
+      self._resource_pickup = None
+      raise e
 
   async def move_picked_up_resource(
     self,