[enh] settings.yml: add use_default_settings option

This change is backward compatible with the existing configurations.

If a settings.yml loaded from an user defined location (SEARX_SETTINGS_PATH or /etc/searx/settings.yml),
then this settings can relied on the default settings.yml with this option:
user_default_settings:True

searx/exceptions.py

@@ -31,3 +31,11 @@ class SearxParameterException(SearxException):
         self.message = message
         self.parameter_name = name
         self.parameter_value = value
+
+
+class SearxSettingsException(SearxException):
+
+    def __init__(self, message, filename):
+        super().__init__(message)
+        self.message = message
+        self.filename = filename