core, bridge, jp, quick_frontend: SàT stop more gracefully if bridge can't be initialised:

- new BridgeInitError and BridgeExceptionNoService are in core.exceptions
    - D-Bus NotSupported is catched in bridge, and launch a BridgeInitError
    - BridgeInitError stop SàT core, jp, and quick_frontends with an explanation message.
fix bug 27

src/core/exceptions.py

@@ -44,3 +44,9 @@ class NotFound(Exception):
 
 class DataError(Exception):
     pass
+
+class BridgeInitError(Exception):
+    pass
+
+class BridgeExceptionNoService(Exception):
+    pass