heater_pc: test template on connect

This allows us to handle template errors early and
show the user visible error output in web ui
Instead of crashing within the reactor.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>

klippy/extras/heater_pc.py

@@ -51,6 +51,8 @@ class HeaterPredictControl:
                 output = self.output
             return output
         heater.set_pc_callback(callback)
+        # Test template
+        self._render(.0)
     def _render(self, eventtime):
         context = self.create_context()
         output = self.template.render(context)