RemoteError._render_traceback_ calls self.render_traceback

rather than an alias

There are two options here:

1. `_render_traceback_`  should *call* render_traceback
2. any subclass that redefines render_traceback must also redefine `_render_traceback_`

I went with 1., which is more efficient code-wise when subclassing RemoteError
(would prevent future cases of this same mistake),
but less efficient execution-wise, because it involves an extra function call.

closes  #2303

IPython/parallel/error.py

@@ -195,7 +195,8 b' class RemoteError(KernelError):'
         return (self.traceback or "No traceback available").splitlines()
     
     # Special method for custom tracebacks within IPython
-    _render_traceback_ = render_traceback
+    def _render_traceback_(self):
+        return self.render_traceback()
 
     def print_traceback(self, excid=None):
         """print my traceback"""