Fix error reporting when the error is reported by the input, not by an op.

theano/gof/cc.py

@@ -1392,7 +1392,8 @@ class _CThunk(object):
                 trace = ()
             try:
                 exc_type, _exc_value, exc_trace = self.error_storage
-                self.position_of_error = self.nodes.index(task)
+                if task in self.nodes:
+                    self.position_of_error = self.nodes.index(task)
                 # this can be used to retrieve the location the Op was declared
                 exc_value = exc_type(_exc_value)
                 exc_value.__thunk_trace__ = trace