basexmpp: Add a message_error event

The "message" event only receives messages with a body, and error
messages don’t necessarily have it. Removing the body requirement from
the "message" event could lean to unhandled conditions in existing code.

docs/event_index.rst

@@ -152,6 +152,13 @@ Event Index
         Makes the contents of message stanzas available whenever one is received. Be
         sure to check the message type in order to handle error messages.
 
+    message_error
+        - **Data:** :py:class:`~slixmpp.Message`
+        - **Source:** :py:class:`BaseXMPP <slixmpp.BaseXMPP>`
+
+        Makes the contents of message stanzas available whenever one is received.
+        Only handler messages with an ``error`` type.
+
     message_form
         - **Data:** :py:class:`~slixmpp.plugins.xep_0004.Form`
         - **Source:** :py:class:`~slixmpp.plugins.xep_0004.xep_0004`

slixmpp/basexmpp.py

@@ -143,6 +143,13 @@ class BaseXMPP(XMLStream):
                      MatchXPath('{%s}message/{%s}body' % (self.default_ns,
                                                           self.default_ns)),
                      self._handle_message))
+
+        self.register_handler(
+            Callback('IMError',
+                     MatchXPath('{%s}message/{%s}error' % (self.default_ns,
+                                                           self.default_ns)),
+                     self._handle_message_error))
+
         self.register_handler(
             Callback('Presence',
                      MatchXPath("{%s}presence" % self.default_ns),
@@ -690,6 +697,12 @@ class BaseXMPP(XMLStream):
             msg['to'] = self.boundjid
         self.event('message', msg)
 
+    def _handle_message_error(self, msg):
+        """Process incoming message error stanzas."""
+        if not self.is_component and not msg['to'].bare:
+            msg['to'] = self.boundjid
+        self.event('message_error', msg)
+
     def _handle_available(self, pres):
         self.roster[pres['to']][pres['from']].handle_available(pres)