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/jp/jp

@@ -53,7 +53,8 @@ from os.path import abspath, basename, dirname
 from optparse import OptionParser
 from sat.tools.jid import JID
 import gobject
-from sat_frontends.bridge.DBus import DBusBridgeFrontend,BridgeExceptionNoService
+from sat_frontends.bridge.DBus import DBusBridgeFrontend
+from sat.core.exceptions import BridgeExceptionNoService, BridgeInitError
 from sat.tools.utils import clean_ustr
 import tarfile
 import tempfile
@@ -74,7 +75,9 @@ class JP(object):
             self.bridge=DBusBridgeFrontend()
         except BridgeExceptionNoService:
             print(_(u"Can't connect to SàT backend, are you sure it's launched ?"))
-            import sys
+            sys.exit(1)
+        except BridgeInitError:
+            print(_(u"Can't init bridge"))
             sys.exit(1)
         self.transfer_data = None