Change error state to be 'error' rather than 'unknown', trace error.

homeassistant/components/sensor/template.py

@@ -12,7 +12,6 @@ import logging
 from homeassistant.helpers.entity import Entity
 from homeassistant.core import EVENT_STATE_CHANGED
 from homeassistant.const import (
-    STATE_UNKNOWN,
     ATTR_FRIENDLY_NAME,
     CONF_VALUE_TEMPLATE,
     ATTR_UNIT_OF_MEASUREMENT)
@@ -24,6 +23,8 @@ _LOGGER = logging.getLogger(__name__)
 
 CONF_SENSORS = 'sensors'
 
+STATE_ERROR = 'error'
+
 
 # pylint: disable=unused-argument
 def setup_platform(hass, config, add_devices, discovery_info=None):
@@ -107,5 +108,6 @@ class SensorTemplate(Entity):
     def update(self):
         try:
             self._state = template.render(self.hass, self._template)
-        except TemplateError:
-            self._state = STATE_UNKNOWN
+        except TemplateError as ex:
+            self._state = STATE_ERROR
+            _LOGGER.error(ex)

tests/components/sensor/test_template.py

@@ -60,4 +60,4 @@ class TestTemplateSensor:
         self.hass.states.set('sensor.test_state', 'Works')
         self.hass.pool.block_till_done()
         state = self.hass.states.get('sensor.test_template_sensor')
-        assert state.state == 'unknown'
+        assert state.state == 'error'