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

frontends/src/bridge/DBus.py

@@ -20,6 +20,7 @@
 from bridge_frontend import BridgeFrontend
 import dbus
 from logging import debug, error
+from sat.core.exceptions import BridgeExceptionNoService, BridgeInitError
 
 from dbus.mainloop.glib import DBusGMainLoop
 DBusGMainLoop(set_as_default=True)
@@ -31,10 +32,6 @@ const_CORE_SUFFIX = ".core"
 const_PLUGIN_SUFFIX = ".plugin"
 
 
-class BridgeExceptionNoService(Exception):
-    pass
-
-
 class DBusBridgeFrontend(BridgeFrontend):
     def __init__(self):
         try:
@@ -48,6 +45,9 @@ class DBusBridgeFrontend(BridgeFrontend):
         except dbus.exceptions.DBusException, e:
             if e._dbus_error_name == 'org.freedesktop.DBus.Error.ServiceUnknown':
                 raise BridgeExceptionNoService
+            elif e._dbus_error_name == 'org.freedesktop.DBus.Error.NotSupported':
+                print u"D-Bus is not launched, please see README to see instructions on how to launch it"
+                raise BridgeInitError
             else:
                 raise e
         #props = self.db_core_iface.getProperties()