ui: send traceback of devel warning to appropriate output stream

If ui.ferr is a command-server channel, traceback should be written to it.

mercurial/ui.py

@@ -994,7 +994,7 b' class ui(object):'
         """issue a developer warning message"""
         msg = 'devel-warn: ' + msg
         if self.tracebackflag:
-            util.debugstacktrace(msg, 2)
+            util.debugstacktrace(msg, 2, self.ferr, self.fout)
         else:
             curframe = inspect.currentframe()
             calframe = inspect.getouterframes(curframe, 2)