core: replaced SkipHistory exception by a key in mess_data:

SkipHistory was skipping all remaining triggers just to skip history, which is not the intented behaviour.
    Now history can be skipped by setting mess_data[u'history'] = C.HISTORY_SKIP, this way we won't skip importants triggers.
    When history is skipped, mess_data[u'extra'][u'history'] will be set to C.HISTORY_SKIP for frontends, so they can inform user that the message is not stored locally.

src/plugins/plugin_xep_0334.py

@@ -21,8 +21,8 @@
 from sat.core.i18n import _, D_
 from sat.core.log import getLogger
 log = getLogger(__name__)
+from sat.core.constants import Const as C
 
-from sat.core import exceptions
 from sat.tools.common import data_format
 
 from wokkel import disco, iwokkel
@@ -30,7 +30,6 @@ try:
     from twisted.words.protocols.xmlstream import XMPPHandler
 except ImportError:
     from wokkel.subprotocols import XMPPHandler
-from twisted.python import failure
 from zope.interface import implements
 from textwrap import dedent
 
@@ -95,9 +94,8 @@ class XEP_0334(object):
         return True
 
     def _receivedSkipHistory(self, mess_data):
-        mess_data[u'extra'][u'history'] == u'skipped'
-        raise failure.Failure(exceptions.SkipHistory())
-
+        mess_data[u'history'] = C.HISTORY_SKIP
+        return mess_data
 
     def messageReceivedTrigger(self, client, message_elt, post_treat):
         """Check for hints in the received message"""