Add validate_table_existence to abstract SettingsManager

The `BackendManager` was using the `validate_table_existence` from its
`SettingsManager` member, but this method was not defined in the abstract
class. It was present in both backends implementations, but there would
be nothing requiring from the abstract parent class.

This is mostly a "formal" quickfix.

aiida/backends/manager.py

@@ -94,6 +94,13 @@ def delete(self, key):
         :raises: `~aiida.common.exceptions.NotExistent` if the settings does not exist
         """
 
+    @abc.abstractmethod
+    def validate_table_existence(self):
+        """Verify that the `DbSetting` table actually exists.
+
+        :raises: `~aiida.common.exceptions.NotExistent` if the settings table does not exist
+        """
+
 
 class BackendManager:
     """Class to manage the database schema and environment."""