core: Handle unpicklable data in dispatch_calls()

Sending just via .call_async() would previously crash the child, now it
generates CallError like intended.

mitogen/core.py

@@ -1330,15 +1330,13 @@ class ExternalContext(object):
         finally:
             fp.close()
 
-    def _dispatch_calls(self):
-        for msg in self.channel:
+    def _dispatch_one(self, msg):
         data = msg.unpickle(throw=False)
         _v and LOG.debug('_dispatch_calls(%r)', data)
         if msg.auth_id not in mitogen.parent_ids:
             LOG.warning('CALL_FUNCTION from non-parent %r', msg.auth_id)
 
         modname, klass, func, args, kwargs = data
-            try:
         obj = __import__(modname, {}, {}, [''])
         if klass:
             obj = getattr(obj, klass)
@@ -1347,7 +1345,12 @@ class ExternalContext(object):
             kwargs.setdefault('econtext', self)
         if getattr(fn, 'mitogen_takes_router', None):
             kwargs.setdefault('router', self.router)
-                msg.reply(fn(*args, **kwargs))
+        return fn(*args, **kwargs)
+
+    def _dispatch_calls(self):
+        for msg in self.channel:
+            try:
+                msg.reply(self._dispatch_one(msg))
             except Exception, e:
                 _v and LOG.debug('_dispatch_calls: %s', e)
                 msg.reply(CallError(e))