Guard emeter accesses to avoid keyerrors (#304)

Raise an exception to inform the caller that update() is needed

kasa/smartdevice.py

@@ -219,6 +219,8 @@ class SmartDevice:
         """Raise an exception if there is no emeter."""
         if not self.has_emeter:
             raise SmartDeviceException("Device has no emeter")
+        if self.emeter_type not in self._last_update:
+            raise SmartDeviceException("update() required prior accessing emeter")
 
     async def _query_helper(
         self, target: str, cmd: str, arg: Optional[Dict] = None, child_ids=None