[IMP] base_external_dbsource: Refactor & Split by source

* Heavily refactor code for reusability
* Split all sources into independent modules
* Add more test coverage
* Add CRUD methods
* Add iterator execute return to roadmap

OCA Transbot updated translations from Transifex

base_external_dbsource/exceptions.py

@@ -0,0 +1,13 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 LasLabs Inc.
+# License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl).
+
+from odoo.exceptions import UserError
+
+
+class ConnectionFailedError(UserError):
+    pass
+
+
+class ConnectionSuccessError(UserError):
+    pass