Make the Qnap sensor more resilient if server is not reachable (#16445)

* add CONF_ALLOW_UNREACHABLE option, retry connection if allowed

* fix linting errors

* Removing the config option, just using PlatformNotReady

homeassistant/components/sensor/qnap.py

@@ -15,6 +15,7 @@ from homeassistant.const import (
     CONF_HOST, CONF_USERNAME, CONF_PASSWORD, CONF_PORT, CONF_SSL, ATTR_NAME,
     CONF_VERIFY_SSL, CONF_TIMEOUT, CONF_MONITORED_CONDITIONS, TEMP_CELSIUS)
 from homeassistant.util import Throttle
+from homeassistant.exceptions import PlatformNotReady
 import homeassistant.helpers.config_validation as cv
 
 REQUIREMENTS = ['qnapstats==0.2.7']
@@ -107,14 +108,9 @@ def setup_platform(hass, config, add_entities, discovery_info=None):
     api = QNAPStatsAPI(config)
     api.update()
 
+    # QNAP is not available
     if not api.data:
-        hass.components.persistent_notification.create(
-            'Error: Failed to set up QNAP sensor.<br />'
-            'Check the logs for additional information. '
-            'You will need to restart hass after fixing.',
-            title=NOTIFICATION_TITLE,
-            notification_id=NOTIFICATION_ID)
-        return False
+        raise PlatformNotReady
 
     sensors = []